Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Feb. 27, 2018	Jun. 30, 2017
Document And Entity Information
Entity Registrant Name	RIBBON COMMUNICATIONS INC.
Entity Central Index Key	0001708055
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		101,928,560
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 357,710,000

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 57,073	$ 31,923
Marketable securities	17,224	61,836
Accounts receivable, net	165,156	53,862
Inventory	21,303	18,283
Other current assets	21,463	12,010
Total current assets	282,219	177,914
Property and equipment, net	24,780	11,741
Intangible assets, net	244,414	30,197
Goodwill	335,716	49,393
Investments	9,031	32,371
Deferred income taxes	8,434	1,542
Other assets	6,289	4,901
Total assets	910,883	308,059
Current liabilities:
Revolving credit facility	20,000	0
Accounts payable	45,851	6,525
Accrued expenses and other	76,380	27,040
Deferred revenue	100,571	43,504
Total current liabilities	242,802	77,069
Long-term debt, related party	22,500	0
Deferred revenue, net of current	14,184	7,188
Deferred income taxes	2,787	3,047
Other long-term liabilities	13,189	1,633
Total liabilities	295,462	88,937
Commitments and contingencies (Note 22)
Stockholders' equity:
Preferred stock, $0.01 par value; 10,000,000 shares authorized at December 31, 2017; 5,000,000 shares authorized at December 31, 2016; none issued and outstanding	0	0
Common stock, 240,000,000 shares authorized, $0.0001 par value, 101,752,856 shares issued and outstanding at December 31, 2017; 120,000,000 shares authorized, $0.001 par value, 49,041,881 shares issued and outstanding at December 31, 2016	10	49
Additional paid-in capital	1,684,768	1,250,744
Accumulated deficit	(1,072,426)	(1,037,174)
Accumulated other comprehensive income	3,069	5,503
Total stockholders' equity	615,421	219,122
Total liabilities and stockholders' equity	$ 910,883	$ 308,059

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.001
Common stock, shares authorized	240,000,000	120,000,000
Common stock, shares issued	101,752,856	49,041,881
Common stock, shares outstanding	101,752,856	49,041,881

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue:
Product	$ 181,119	$ 146,381	$ 141,913
Service	148,823	106,210	107,121
Total revenue	329,942	252,591	249,034
Cost of revenue:
Product	70,250	47,367	50,460
Service	58,196	37,613	36,917
Total cost of revenue	128,446	84,980	87,377
Gross profit	201,496	167,611	161,657
Operating expenses:
Research and development	101,481	72,841	77,908
Sales and marketing	83,403	68,539	72,841
General and administrative	47,642	35,948	39,846
Acquisition- and integration-related	14,763	1,152	131
Restructuring	9,436	2,740	2,148
Total operating expenses	256,725	181,220	192,874
Loss from operations	(55,229)	(13,609)	(31,217)
Interest income, net	263	769	207
Other income, net	1,274	1,424	1,122
Loss before income taxes	(53,692)	(11,416)	(29,888)
Income tax benefit (provision)	18,440	(2,516)	(2,007)
Net loss	$ (35,252)	$ (13,932)	$ (31,895)
Loss per share:
Basic (in dollars per share)	$ (0.60)	$ (0.28)	$ (0.64)
Diluted (in dollars per share)	$ (0.60)	$ (0.28)	$ (0.64)
Shares used to compute loss per share:
Basic (in shares)	58,822	49,385	49,560
Diluted (in shares)	58,822	49,385	49,560

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net loss	$ (35,252)	$ (13,932)	$ (31,895)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments	(1,940)	54	9
Unrealized gain (loss) on available-for-sale marketable securities, net of reclassification adjustments for realized amounts	146	51	(15)
Employee retirement benefits	(578)	0	0
Other comprehensive income (loss), net of tax	(2,372)	105	(6)
Comprehensive loss, net of tax	$ (37,624)	$ (13,827)	$ (31,901)

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	GENBAND
Balance at Dec. 31, 2014	$ 240,350	$ 49	$ 1,226,226	$ (991,347)	$ 5,422
Balance (in shares) at Dec. 31, 2014		49,357,033
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with employee stock purchase plan	2,378		2,378
Issuance of common stock in connection with employee stock purchase plan (in shares)		233,659
Exercise of stock options	1,757		1,757
Exercise of stock options (in shares)		155,478
Vesting of restricted stock awards	1	$ 1
Vesting of restricted stock awards (in shares)		491,739
Vesting of performance-based stock awards	0
Vesting of performance-based stock awards (in shares)		45,901
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(2,344)		(2,344)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(167,634)
Repurchase of common stock	(7,917)	$ (1)	(7,916)
Repurchase of common stock (in shares)		(642,387)
Stock-based compensation expense	21,699		21,699
Other Comprehensive Income (Loss), Net of Tax	(6)				(6)
Reclassification of equity to liability for equity awards converted to cash bonuses	(997)
Net loss	(31,895)			(31,895)
Balance at Dec. 31, 2015	223,026	$ 49	1,240,803	(1,023,242)	5,416
Balance (in shares) at Dec. 31, 2015		49,473,789
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with employee stock purchase plan	1,360		1,360
Issuance of common stock in connection with employee stock purchase plan (in shares)		225,031
Exercise of stock options	153		153
Exercise of stock options (in shares)		23,070
Vesting of restricted stock awards	0	$ 1	(1)
Vesting of restricted stock awards (in shares)		792,773
Vesting of performance-based stock awards	0
Vesting of performance-based stock awards (in shares)		18,438
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(1,810)		(1,810)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(231,620)
Repurchase of common stock	(9,530)	$ (1)	(9,529)
Repurchase of common stock (in shares)		(1,259,600)
Stock-based compensation expense	19,768		19,768
Other Comprehensive Income (Loss), Net of Tax	87				87
Net loss	(13,932)			(13,932)
Balance at Dec. 31, 2016	$ 219,122	$ 49	1,250,744	(1,037,174)	5,503
Balance (in shares) at Dec. 31, 2016	49,041,881	49,041,881
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with employee stock purchase plan	$ 1,252		1,252
Issuance of common stock in connection with employee stock purchase plan (in shares)		249,621
Exercise of stock options	617		617
Exercise of stock options (in shares)		105,688
Vesting of restricted stock awards	0
Vesting of restricted stock awards (in shares)		2,160,553
Vesting of performance-based stock awards	0
Vesting of performance-based stock awards (in shares)		145,357
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(7,523)		(7,523)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(807,952)
Shares issued as consideration in connection with acquisition of GENBAND		$ 5	413,977			$ 413,982
Shares issued as consideration in connection with acquisition of GENBAND (in shares)						50,857,708
Stock-based compensation expense	25,657		25,657
Reclassification between Common stock and Additional paid-in capital to record change in par value of common stock	0	(44)	44
Other Comprehensive Income (Loss), Net of Tax	(2,434)				(2,434)
Net loss	(35,252)			(35,252)
Balance at Dec. 31, 2017	$ 615,421	$ 10	$ 1,684,768	$ (1,072,426)	$ 3,069
Balance (in shares) at Dec. 31, 2017	101,752,856	101,752,856

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (35,252)	$ (13,932)	$ (31,895)
Adjustments to reconcile net loss to cash flows provided by operating activities:
Depreciation and amortization of property and equipment	8,486	7,970	11,961
Amortization of intangible assets	17,112	7,500	7,107
Stock-based compensation	25,657	19,768	21,699
Impairment of intangible assets	5,471	0	0
Deferred income taxes	(20,361)	1,088	752
Other	(1,340)	(1,265)	(784)
Changes in operating assets and liabilities:
Accounts receivable	(30,759)	(851)	11,369
Inventory	5,786	4,858	(1,001)
Other operating assets	269	506	4,915
Accounts payable	13,415	(821)	(1,257)
Accrued expenses and other long-term liabilities	(4,263)	(7,778)	(4,134)
Deferred revenue	23,859	2,149	1,137
Net cash provided by operating activities	8,080	19,192	19,869
Cash flows from investing activities:
Purchases of property and equipment	(3,999)	(4,626)	(7,792)
Business acquisitions, net of cash acquired	(42,951)	(20,669)	(10,897)
Purchases of marketable securities	(28,731)	(78,528)	(54,772)
Sale/maturities of marketable securities	96,112	75,178	67,980
Proceeds from the sale of intangible assets	576	1,298	896
Net cash provided by (used in) investing activities	21,007	(27,347)	(4,585)
Cash flows from financing activities:
Borrowings under revolving line of credit	15,500	0	0
Principal payments on revolving line of credit	(13,500)	0	0
Principal payments of capital lease obligations	(99)	(43)	(76)
Payment of debt issuance costs	(731)	0	0
Proceeds from the sale of common stock in connection with employee stock purchase plan	1,252	1,360	2,378
Proceeds from the exercise of stock options	617	153	1,757
Payment of tax withholding obligations related to net share settlements of restricted stock awards	(7,523)	(1,810)	(2,344)
Repurchase of common stock	0	(9,530)	(7,917)
Net cash used in financing activities	(4,484)	(9,870)	(6,202)
Effect of exchange rate changes on cash and cash equivalents	547	(163)	(128)
Net increase (decrease) in cash and cash equivalents	25,150	(18,188)	8,954
Cash and cash equivalents, beginning of year	31,923	50,111	41,157
Cash and cash equivalents, end of period	57,073	31,923	50,111
Supplemental disclosure of cash flow information:
Interest paid	317	41	64
Income taxes paid	2,290	1,249	1,430
Income tax refunds received	274	511	357
Supplemental disclosure of non-cash investing activities:
Capital expenditures incurred, but not yet paid	1,043	277	375
Property and equipment acquired under capital lease	0	36	137
Business acquisition purchase consideration - common stock issued	413,982	0	0
Business acquisition purchase consideration - note issued to selling equity holders	22,500	0	0
Supplemental disclosure of non-cash financing activities:
Total fair value of restricted stock awards, restricted stock units, performance-based stock awards and performance-based stock units on date vested	$ 20,515	$ 10,376	$ 9,138

NATURE OF THE BUSINESS	12 Months Ended
NATURE OF THE BUSINESS	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
NATURE OF THE BUSINESS	NATURE OF THE BUSINESS Ribbon is a leading provider of network communications solutions to telecommunications, wireless and cable service providers and enterprises of all sizes across industry verticals. With over 1,000 customers around the globe, including some of the largest telecommunications service providers and enterprises in the world, Ribbon enables service providers and enterprises to modernize their communications networks and provide secure RTC solutions to their customers and employees. By securing and enabling reliable and scalable IP networks, Ribbon helps service providers and enterprises adopt the next generation of software-based virtualized and cloud communications technologies to drive new, incremental revenue while protecting their existing revenue streams. Ribbon's solutions provide a secure way for its customers to connect and leverage multivendor, multiprotocol communications systems and applications across their networks and the cloud, around the world and in a rapidly changing ecosystem of IP-enabled devices such as smartphones and tablets. In addition, Ribbon's solutions secure the evolution to cloud-based delivery of UC solutions - both for service providers transforming to a cloud-based network and for enterprises using cloud-based UC. Ribbon goes to market through both direct sales and indirect channels globally, leveraging the assistance of resellers, and provides ongoing support to its customers through a global services team with experience in design, deployment and maintenance of some of the world's largest IP networks. The Merger with GENBAND (see Note 2) was completed in October 2017. As a result of the Merger, Ribbon believes it is better positioned to enable network transformations to IP and to cloud-based networks for service providers and enterprise customers worldwide, with a broader and deeper sales footprint, increased ability to invest in growth, more efficient and effective research and development, and a comprehensive RTC product offering.

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation The consolidated financial statements have been prepared in United States dollars, in accordance with accounting principles generally accepted in the United States ("GAAP"). On October 27, 2017 (the "Merger Date"), Sonus Networks, Inc. ("Sonus") consummated an acquisition as specified in an Agreement and Plan of Merger (the “Merger Agreement”) with Solstice Sapphire Investments, Inc. ("NewCo") and certain of its wholly-owned subsidiaries, GENBAND Holdings Company, GENBAND Inc. and GENBAND II, Inc. (collectively, "GENBAND") pursuant to which, following a series of merger transactions (collectively, the "Merger"), Sonus and GENBAND each became a wholly-owned subsidiary of NewCo, with Sonus deemed the acquirer in the transaction for accounting purposes. Subsequently, on November 28, 2017, the Company changed its name to "Ribbon Communications Inc." The consolidated financial statements of the Company represent the consolidated financial statements of Sonus, prior to the Merger Date, and the consolidated financial statements of Ribbon, on and after the Merger Date. The financial results of GENBAND are included in Ribbon's consolidated financial statements beginning on the Merger Date. Significant Accounting Policies Principles of Consolidation The accompanying consolidated financial statements include the accounts of Ribbon and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated in consolidation. Use of Estimates and Judgments The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting periods. Significant estimates and judgments relied upon in preparing these consolidated financial statements include accounting for business combinations, revenue recognition for multiple element arrangements, inventory valuations, assumptions used to determine the fair value of stock-based compensation, intangible assets and goodwill valuations, legal contingencies and recoverability of Ribbon's net deferred tax assets and the related valuation allowances. Ribbon regularly assesses these estimates and records changes in estimates in the period in which they become known. Ribbon bases its estimates on historical experience and various other assumptions that it believes to be reasonable under the circumstances. Actual results could differ from those estimates. Reclassifications Certain reclassifications, not affecting previously reported net loss, have been made to the previously issued financial statements to conform to the current period presentation. Business Combinations The Company recognizes identifiable assets acquired and liabilities assumed at their acquisition date fair values. Goodwill as of the acquisition date is measured as the excess of consideration transferred over the net of the acquisition date fair values of the assets acquired and the liabilities assumed and represents the expected future economic benefits arising from other assets acquired in the business combination that are not individually identified and separately recognized. While the Company uses its best estimates and assumptions as part of the purchase price allocation process to accurately value assets acquired and liabilities assumed at the acquisition date, its estimates are inherently uncertain and subject to refinement. As a result, during the measurement period, which may be up to one year from the acquisition date, the Company records adjustments to the assets acquired and liabilities assumed with the corresponding offset to goodwill to the extent that it identifies adjustments to the preliminary purchase price allocation. Upon the conclusion of the measurement period or final determination of the values of assets acquired and liabilities assumed, whichever comes first, any subsequent adjustments are recorded to the consolidated statements of operations. Revenue Recognition The Company recognizes revenue from sales when persuasive evidence of an arrangement exists, delivery has occurred, the sale price is fixed or determinable, and collectability of the related receivable is reasonably assured. In instances where customer acceptance is required, revenue is deferred until the acceptance has been achieved. When fees for products or services are not fixed and determinable, the Company defers the recording of receivables, deferred revenue and revenue until such time as the fees become due or are collected. Revenue from maintenance and support services is recognized ratably over the service period. Maintenance revenue is deferred until the associated product is accepted by the customer and all other revenue recognition criteria have been met. Maintenance and support services include telephone support, return and repair support and unspecified rights to product upgrades and enhancements. Revenue from other professional services is typically recognized as the services are delivered if all other revenue recognition criteria have been met. The Company's products typically have both software and non-software components that function together to deliver the products' essential functionality. In addition, hardware sold generally cannot be used apart from the software. Therefore, the Company considers its principal products to be both software and hardware-related. Many of the Company's sales involve multiple element arrangements that include product, maintenance and various professional services. The Company recognizes revenue in accordance with the provisions of Accounting Standards Codification ("ASC") 605-25, Revenue Recognition - Multiple-Element Arrangements ("ASC 605-25") for transactions that include both hardware and software components. The Company recognizes revenue from stand-alone software sales under the software revenue recognition guidance in ASC 985-605, Software - Revenue Recognition ("ASC 985-605"). The Company limits the amount of revenue recognized for delivered elements to the amount that is not contingent on the future delivery of products or services, future performance obligations, or subject to customer-specific return or refund privileges. For multiple-element arrangements that include both software-only products and non-software products, the Company allocates the total arrangement consideration to the software-only deliverables as a group and to the individual non-software deliverables based on their relative selling prices. If an undelivered element (such as maintenance and support services) relates to both the software-only and non-software deliverables, the Company bifurcates the consideration allocated to the undelivered element (such as maintenance and support services) into a non-software component and the software-only component using the relative selling price method. The consideration allocated to the non-software and software-only deliverables is recognized in accordance with the guidance as discussed in this note. Under ASC 985-605, revenue for any undelivered elements that are considered not essential to the functionality of the product and for which VSOE has been established is deferred and recognized upon delivery utilizing the residual method. If the Company has undelivered product for which VSOE has not been established, it defers all revenue on the entire arrangement until VSOE is established or until such elements are delivered, provided that all other revenue recognition criteria are met. If the Company has undelivered services for which VSOE has not been established, the entire arrangement is recognized as revenue over the longest remaining service period from the point in time that all services have commenced and all products have been delivered, provided that all other revenue recognition criteria are met. For transactions that include multiple elements, arrangement consideration is allocated to each element based on the relative selling prices of all of the elements in the arrangement using the fair value hierarchy as required by ASC 605-25. The Company establishes VSOE based upon the price charged when the same element is sold separately or established by management having the relevant pricing authority. The Company has VSOE for certain of its maintenance and support services and certain professional services. When VSOE exists, it is used to determine the selling price of a deliverable. The Company has not been able to establish VSOE of its products, for certain of its services and for certain maintenance offerings because the Company has not sold such products or services on a stand-alone basis, has not priced its products or services within a narrow range, or has limited sales history. When VSOE is not established, the Company attempts to establish the selling price of each element based on TPE. The Company's solution typically differs from that of its peers as there are no similar or interchangeable competitor products or services. The Company's various product, service and maintenance offerings contain a significant level of unique features and functionality and therefore, comparable pricing of competitors' products and services with similar functionality cannot be obtained. Accordingly, the Company is not able to determine TPE for its products or services. When the Company is unable to establish selling price using VSOE or TPE, the Company uses ESP in its allocation of arrangement consideration for the relevant deliverables. The objective of ESP is to determine the price at which the Company would transact a sale if a product or service was sold on a stand-alone basis. The Company determines ESP for its products and certain services by considering multiple factors including, but not limited to, overall market conditions, including geographic or regional-specific market factors, profit objectives and historical pricing practices for such deliverables. The determination of ESP is a formal process within the Company that includes review and approval by the Company's management. Deferred revenue typically includes customer deposits and amounts associated with partial product shipments and maintenance or service contracts. Deferred revenue expected to be recognized as revenue more than one year subsequent to the balance sheet date is reported as a component of long-term liabilities in the consolidated balance sheets. The Company defers recognition of incremental direct costs, such as cost of goods, third-party installations and commissions, until recognition of the related revenue. Such costs are classified as current assets if the deferred revenue is initially classified as current and noncurrent assets if the related deferred revenue is initially classified as long-term. The Company excludes any taxes assessed by a governmental authority that are directly imposed on a revenue-producing transaction (i.e., sales, use and value added) from its revenue and costs. Reimbursement received for out-of-pocket expenses and shipping costs is recorded as revenue. The Company sells the majority of its products directly to its end customers. For products sold to resellers and distributors, the Company recognizes revenue on a sell-through basis. Financial Instruments The carrying amounts of Ribbon's financial instruments, which include cash equivalents, investments, accounts receivable and accounts payable, approximate their fair values. All investments in marketable securities are classified as available-for-sale and are reported at fair value, with unrealized gains and losses excluded from earnings and reported, net of tax, in Accumulated other comprehensive loss, which is a component of stockholders' equity. Unrealized losses that are determined to be other-than-temporary, based on current and expected market conditions, are recognized in earnings. Declines in fair value determined to be credit-related are charged to earnings. The cost of marketable securities sold is determined by the specific identification method. Financial instruments with remaining maturities or that are due within one year from the balance sheet date are classified as current. Financial instruments with remaining maturities or that are payable more than one year from the balance sheet date are classified as noncurrent. Cash and Cash Equivalents Cash equivalents are stated at fair value. Cash equivalents are liquid securities that have remaining maturities of three months or less at the date of purchase. Restricted Cash The Company classifies as restricted cash all cash pledged as collateral to secure long-term obligations and all cash whose use is otherwise limited by contractual provisions. Restricted cash is recorded within other current assets on the consolidated balance sheet. Foreign Currency Translation For foreign subsidiaries where the functional currency is the local currency, assets and liabilities are translated into U.S. dollars at the current exchange rate on the balance sheet date. Revenue and expenses are translated at average rates of exchange prevailing during each period. Translation adjustments for these subsidiaries are included in Accumulated other comprehensive loss. For foreign subsidiaries where the functional currency is the U.S. dollar, monetary assets and liabilities are translated into U.S. dollars at the current exchange rate on the balance sheet date. Nonmonetary assets and liabilities are remeasured into U.S. dollars at historical exchange rates. Revenue and expense items are translated at average rates of exchange prevailing during each period. Realized and unrealized foreign currency gains and losses arising from transactions denominated in currencies other than the subsidiary's functional currency are reflected in earnings with the exception of intercompany transactions considered to be of a long-term investment nature. Effective on the Merger Date, the Company began to record its foreign currency transaction gains (losses) as a component of Other income (expense), net. The Company did not reclassify amounts previously recorded within General and administrative expenses as the amounts were not material to the consolidated results of the Company. The Company recognized net transaction gains of $0.7 million for the year ended December 31, 2017 and net transaction losses of $0.3 million for the year ended December 31, 2016 and $0.4 million for the year ended December 31, 2015. Inventory Inventory is recorded at the lower of cost or market value using the first-in, first-out convention. The Company reduces the carrying value of inventory for those items that are potentially excess, obsolete or slow-moving based on changes in customer demand, technology developments or other economic factors. Ribbon writes down evaluation equipment at the time of shipment to its customers, as it is probable that the inventory value will not be realized. Deferred product costs represent deferred cost of revenue for product shipments to customers prior to satisfaction of Ribbon's revenue recognition criteria. The Company classifies inventory that is not expected to be consumed within one year from the balance sheet date as noncurrent and includes such inventory as a component of Other assets. Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation. Expenditures for maintenance and repairs are charged to expense as incurred. Depreciation is computed using the straight-line method over the estimated useful lives of the related assets, which range from two to five years. Leasehold improvements are amortized over the lesser of the lease term or five years. When an asset is sold or retired, the cost and related accumulated depreciation or amortization are eliminated, and the resulting gain or loss, if any, is recognized in income (loss) from operations in the consolidated statement of operations. The Company reviews property and equipment for impairment in the same manner as intangible assets discussed below. Software development costs associated with internal use software are incurred in three stages of development: the preliminary project stage, the application development stage and the post-implementation stage. Costs incurred during the preliminary project and post-implementation stages are expensed as incurred. Certain qualifying costs incurred during the application development stage are capitalized as property and equipment. Internal use software is amortized on a straight-line basis over its estimated useful life of three years, beginning when the software is ready for its intended use. Intangible Assets and Goodwill Intangible assets are comprised of certain intangible assets arising from the Merger, as well as previous acquisitions. These intangible assets include a combination of in-process research and development, developed technology, customer relationships, trade names, and internal use software. Intangible assets are reviewed for impairment when events or changes in circumstances indicate that their carrying amounts may not be recoverable based upon the estimated undiscounted cash flows. Recoverability of intangible assets with estimated lives and other long-lived assets is measured by a comparison of the carrying amount of an asset or asset group to future net undiscounted cash flows expected to be generated by the asset or asset group. If these comparisons indicate that an asset is not recoverable, the Company will recognize an impairment loss for the amount by which the carrying value of the asset or asset group exceeds the related estimated fair value. Estimated fair value is based on either discounted future operating cash flows or appraised values, depending on the nature of the asset. The Company amortizes its intangible assets over their respective useful lives, with the exception of in-process research and development, which has an indefinite life until the product is generally available, at which time such asset is typically reclassified to developed technology, and the Company begins to amortize this asset. See Note 9 for additional information regarding the Company's intangible assets. Goodwill is recorded when the consideration for an acquisition exceeds the fair value of net tangible and identifiable intangible assets acquired. Goodwill is not amortized, but instead is tested for impairment at least annually, or more frequently if indicators of potential impairment exist, by comparing the fair value of the Company's reporting unit to its carrying value. The Company's annual testing for impairment of goodwill is completed as of November 30. The Company operates as a single operating segment with one reporting unit and consequently evaluates goodwill for impairment based on an evaluation of the fair value of the Company as a whole. The Company performed its step one assessments for each of the years ended December 31, 2017, 2016 and 2015 and determined each year that its market capitalization was significantly in excess of its carrying value and accordingly, there was no impairment of goodwill. Other Assets Other assets are primarily comprised of the long-term portion of deferred cost of goods sold, prepaid expenses and deposits. Stock-Based Compensation The Company's stock-based compensation cost is measured at the grant date based on the fair value of the award and is recognized as expense over the requisite service period, which generally represents the vesting period, and includes an estimate of the awards that will be forfeited. The Company uses the Black-Scholes valuation model for estimating the fair value on the date of grant of stock options. The fair value of stock option awards is affected by the Company's stock price as well as valuation assumptions, including the volatility of Ribbon's stock price, expected term of the option, risk-free interest rate and expected dividends. In 2015, the Company began to grant performance-based stock units ("PSUs") that include a market condition to certain of its executives. The Company uses a Monte Carlo simulation approach to model future stock price movements based upon the risk-free rate of return, the volatility of each entity and the pair-wise covariance between each entity. These results are then used to calculate the grant date fair values of the PSUs. Research and Development Costs Research and development costs are expensed as incurred. Concentrations of Credit Risk The financial instruments that potentially subject Ribbon to concentrations of credit risk are cash, cash equivalents, investments and accounts receivable. The Company's cash equivalents and investments were managed by one financial institution at both December 31, 2017 and 2016. Historically, the Company has not experienced significant losses due to such bank depository concentration. Certain components and software licenses from third parties used in Ribbon's products are procured from single sources of supply. The failure of a supplier, including a subcontractor, to deliver on schedule could delay or interrupt Ribbon's delivery of products and thereby materially adversely affect Ribbon's revenues and operating results. Advertising Costs Advertising costs are expensed as incurred and included as a component of Sales and marketing expense in the Company's consolidated statements of operations. Advertising expenses were $0.3 million for the year ended December 31, 2017, $0.1 million for the year ended December 31, 2016 and $0.9 million for the year ended December 31, 2015. Operating Segments The Company operates in a single segment, as the chief operating decision maker makes decisions and assesses performance at the company level. Operating segments are identified as components of an enterprise about which separate discrete financial information is utilized for evaluation by the chief operating decision maker in making decisions regarding resource allocation and assessing performance. To date, the chief operating decision maker has made such decisions and assessed performance at the company level, as one segment. The Company's chief operating decision maker is its President and Chief Executive Officer. Loss Contingencies and Reserves Ribbon is subject to ongoing business risks arising in the ordinary course of business, including legal claims, that affect the estimation process of the carrying value of assets, the recording of liabilities and the possibility of various loss contingencies. An estimated loss contingency is accrued when it is probable that a liability has been incurred or an asset has been impaired and the amount of loss can be reasonably estimated. Ribbon regularly evaluates current information available to determine whether such amounts should be adjusted and records changes in estimates in the period they become known. An allowance for doubtful accounts is estimated based on the Company's assessment of the collectability of specific customer accounts. Ribbon accrues for royalties for technology that it licenses from vendors based on established royalty rates and usage. In certain cases, Ribbon has been contacted by third parties who claim that Ribbon's products infringe on certain intellectual property of a third party. Ribbon evaluates these claims and accrues amounts when it is probable that the obligation has been incurred and the amounts are reasonably estimable. Accounting for Income Taxes Deferred tax assets and liabilities are recognized for the expected future consequences of events that have been reflected in the consolidated financial statements. Deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax basis of assets and liabilities and operating loss carryforwards, using tax rates expected to be in effect for the years in which the differences are expected to reverse. The Company records valuation allowances to reduce deferred income tax assets to the amount that is more likely than not to be realized. The Company has provided for income taxes on the undistributed earnings of its non-U.S. subsidiaries as of December 31, 2017, with the exception of the Company's Irish subsidiary, as the Company does not plan to permanently reinvest these amounts outside the United States. The repatriation of the undistributed earnings would result in withholding taxes imposed on the repatriation. Consequently, the Company has recorded a tax liability of $3.2 million, primarily consisting of withholding and distribution taxes, relating to undistributed earnings from these subsidiaries as of December 31, 2017. Had the earnings of the Irish subsidiary been determined to not be permanently reinvested outside the U.S., no additional deferred tax liability would be required due to no withholding taxes or income tax expense being imposed on such repatriation. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, no amount of the benefit attributable to the position is recognized. The tax benefit to be recognized of any tax position that meets the more likely than not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. Defined Benefit Plans In connection with the Merger, the Company assumed defined benefit plans for some of GENBAND's employees at various international locations. The Company recognizes retirement benefit assets or liabilities in the consolidated balance sheets reflecting the funded status of pension and other retirement benefit plans. Retirement benefit assets and liabilities are adjusted for the difference between the benefit obligations and the plan assets at fair value (measured at year-end), with the offset recorded directly to stockholders' equity through accumulated other comprehensive income (loss), net of tax. The amount recorded in stockholders' equity represents the after-tax unamortized actuarial gains or losses, unamortized transition obligations and unamortized prior service costs. The amounts included in the Company's consolidated financial statements as of and for the year ended December 31, 2017 represent the activity for the period from the Merger Date to December 31, 2017. Recent Accounting Pronouncements In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), its new standard on revenue from contracts with customers, along with additional ASUs which, among other things, clarified the implementation of the new revenue guidance and delayed the adoption by one year, to January 1, 2018 (collectively, the "New Revenue Standard" or "ASC 606"). The New Revenue Standard outlines a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. The core principle of the revenue model is that an entity recognizes revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In applying the revenue model to contracts within its scope, an entity identifies the contract(s) with a customer, identifies the performance obligations in the contract, determines the transaction price, allocates the transaction price to the performance obligations in the contract and recognizes revenue when (or as) the entity satisfies a performance obligation. Certain of the New Revenue Standard's provisions also apply to transfers of nonfinancial assets, including in-substance nonfinancial assets that are not an output of an entity’s ordinary activities (i.e., property, plant and equipment; real estate; or intangible assets). Existing accounting guidance applicable to these transfers has been amended or superseded. Effective January 1, 2018, the Company adopted the New Revenue Standard using the modified retrospective option and has identified the necessary changes to its policies, processes, systems and controls. However, due to the recent acquisition of GENBAND, the Company has not yet completed all of its internal control procedures. Under the modified retrospective method, the Company is applying the New Revenue Standard to all contracts not yet completed as of January 1, 2018, recognizing in beginning Accumulated deficit an adjustment for the cumulative effect of the change and providing additional disclosures comparing results to those as if the Company was still following the previous accounting standards. Currently, the Company does not have VSOE for certain elements in software bundled arrangements, which results in revenue being recognized ratably over the longest performance period. The majority of the Company's transition adjustment will be related to these arrangements. The Company currently expects that the adjustment to decrease Accumulated deficit as a result of this change will not exceed $15 million. Additionally, the Company expects to capitalize certain commission costs resulting directly from securing contracts which are currently expensed. The Company expects this to result in the capitalization of approximately $1 million of previously expensed commissions expense. In May 2017, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting ("ASU 2017-09"), which amends the scope of modification accounting for share-based payment arrangements such that an entity would not apply modification accounting if the fair value, vesting conditions and classification of the awards are the same immediately before and after the modification. ASU 2017-09 is effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. The Company does not expect the adoption of ASU 2017-09 will have a material impact on its consolidated financial statements. In March 2017, the FASB issued ASU 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post Retirement Benefit Cost ("ASU 2017-07"). ASU 2017-07 amends the requirements in ASC 715 to require entities to disaggregate the current-service-cost component from the other components of net benefit cost (the "other components") and include it with other current compensation costs for related employees, present the other components elsewhere in the income statement and outside of income from operations if such a subtotal is presented and disclose the income statement lines that contain the other components if they are not presented on appropriately described separate lines. ASU 2017-07 is effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. The Company does not expect the adoption of ASU 2017-07 will have a material impact on our consolidated financial statements. In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ("ASU 2017-04"), which removes the requirement to compare the implied fair value of goodwill with its carrying amount as part of step 2 of the goodwill impairment test by instead comparing the fair value of a reporting unit with its carrying amount and recognizing an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value. ASU 2017-04 also clarifies the treatment of foreign currency translation adjustments to reporting units and the income tax effects on goodwill impairment loss measurement. ASU 2017-04 is effective prospectively for the Company beginning January 15, 2020, with early adoption permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company elected to early-adopt ASU 2017-04; such early adoption did not have a material impact on its consolidated financial results. In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory ("ASU 2016-16"), which removes the prohibition in ASC 740, Income Taxes, against the immediate recognition of the current and deferred income tax effects of intra-entity transfers of assets other than inventory. ASU 2016-16 is effective for us beginning January 1, 2019 for both interim and annual reporting periods. The Company does not believe that the adoption of this standard will have a material impact on our consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments ("ASU 2016-15"), which adds or clarifies guidance on eight cash flow issues, including debt prepayment or debt extinguishment costs, settlement of zero-coupon debt instruments or certain other debt instruments, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, proceeds from the settlement of corporate-owned life insurance policies, distributions received from equity method investees, beneficial interests in securitization transactions and separately identifiable cash flows and application of the predominance principle. ASU 2016-15 is effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. Entities must apply the guidance retrospectively to all periods presented but may apply it prospectively from the earliest date practicable if retrospective application would be impracticable. The Company does not expect the adoption of ASU 2016-15 will have a material impact on its consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"), which adds an impairment model that is based on expected losses rather than incurred losses. Under ASU 2016-13, an entity recognizes as an allowance its estimate of expected credit losses, which the FASB believes will result in more timely recognition of such losses. ASU 2016-13 is effective for the Company beginning January 1, 2020 for both interim and annual reporting periods, with early adoption permitted. The Company does not expect the adoption of ASU 2016-13 will have a material impact on its consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting ("ASU 2016-09"), which simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures and statutory tax withholding requirements, as well as classification in the statement of cash flows. ASU 2016-09 became effective for the Company beginning January 1, 2017 for both interim and annual reporting periods. Under ASU 2016-09, the Company will now recognize unrealized excess tax benefits. Due to the full valuation allowance on the Company's federal and state income taxes, the adoption of ASU 2016-09 did not have a material impact on its accounting for income taxes. Without the valuation allowance, the Company would have recognized an increased deferred tax asset approximating $5 million. The Company has elected to continue to apply forfeiture rates to the expense attribution related to stock options, restricted stock awards and restricted stock units, as the Company believes that such continued application results in more accurate expense attribution over the life of these equity grants. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Section A - Leases: Amendments to the FASB Accounting Standards Codification ("ASU 2016-02"), its new standard on accounting for leases. ASU 2016-02 introduces a lessee model that brings most leases onto the balance sheet. ASU 2016-02 eliminates the current GAAP requirement for an entity to use bright-line tests in determining lease classification. ASU 2016-02 is effective for the Company for both interim and annual periods beginning January 1, 2019. Upon adoption of ASU 2016-02, the Company will recognize lease obligations for the right to use these assets in connection with its existing lease agreements. The Company is currently assessing the potential impact of the adoption of ASU 2016-02 on its consolidated financial statements and accordingly, such amounts to be recognized on the balance sheet have yet to be determined. In July 2015, the FASB issued ASU 2015-11, Simplifying the Measurement of Inventory ("ASU 2015-11"), which simplifies the measurement of inventory by requiring entities to measure most inventory at the lower of cost and net realizable value, replacing the previous requirement to measure most inventory at the lower of cost or market. ASU 2015-11 does not apply to inventories that are measured by using either the last-in, first-out method or the retail inventory method. ASU 2015-11 became effective for the Company for both interim and annual reporting periods beginning January 1, 2017. The adoption of ASU 2015-11 did not have a material impact on its consolidated financial statements.

BUSINESS ACQUISITONS

12 Months Ended

Dec. 31, 2017

Business Combinations [Abstract]

Business Acquisitions

BUSINESS ACQUISITIONS

GENBAND Merger

On October 27, 2017, Sonus consummated an acquisition as specified in the Merger Agreement with NewCo and GENBAND such that, following the Merger, Sonus and GENBAND each became a wholly-owned subsidiary of NewCo, with Sonus deemed the acquirer in the transaction for accounting purposes. On November 28, 2017, the Company changed its name to "Ribbon Communications Inc."

Prior to the Merger, GENBAND was a Cayman Islands exempted company limited by shares that was formed on April 7, 2010. Through its wholly owned operating subsidiaries, GENBAND created rapid communications and applications for service providers, enterprises, independent software vendors, system integrators and developers globally. A majority of GENBAND's shares were held by JPMorgan Chase & Co. and managed by One Equity Partners ("OEP"). GENBAND shares were not listed on an exchange or quoted on any automated services, and there was no established trading market for GENBAND shares.

The Company believes that Sonus' and GENBAND's complementary products, solutions and strategies position the combined company to deliver comprehensive solutions to service providers and enterprises migrating to a virtualized all-IP environment in an expanded customer and global footprint.

Pursuant to the Merger Agreement, NewCo issued 50.9 million shares of Sonus common stock to the GENBAND equity holders, with the number of shares issued in the aggregate to the GENBAND equity holders equal to the number of shares of Sonus common stock outstanding immediately prior to the closing date of the Merger, such that former stockholders of Sonus would own approximately 50%, and former shareholders of GENBAND would own approximately 50%, of the shares of NewCo common stock issued and outstanding immediately following the consummation of the Merger.

In addition, NewCo repaid GENBAND’s long-term debt, including both principal and unpaid interest, to a related party of GENBAND totaling $48 million and repaid GENBAND’s management fees due to an affiliate of OEP totaling $10.3 million. NewCo also issued a promissory note for $22.5 million to certain GENBAND equity holders.

NewCo assumed the liability under GENBAND's revolving credit facility with Silicon Valley Bank (the "GENBAND facility"), which had outstanding borrowings and letters of credit totaling $17.9 million and $2.9 million, respectively, at October 27, 2017. At October 27, 2017, the outstanding borrowings had an average interest rate of 4.67%.

The Merger has been accounted for as a business combination and the financial results of GENBAND have been included in the Company's consolidated financial statements for the period subsequent to its acquisition.

As of December 31, 2017, the valuation of acquired assets, identifiable intangible assets and certain assumed liabilities is preliminary. The Company is still in the process of investigating the facts and circumstances existing as of the Merger Date in order to finalize its valuation. The Company expects to finalize the valuation of the assets acquired and liabilities assumed in the third quarter of 2018.

A summary of the preliminary allocation of the purchase consideration for GENBAND is as follows (in thousands):



Fair value of consideration transferred:
Cash consideration:
Repayment of GENBAND long-term debt and accrued interest, related party	$	47,973
Payment of GENBAND management fees due to majority shareholder	10,302
Less cash acquired	(15,324		)
Net cash consideration	42,951
Fair value of Sonus stock issued	413,982
Promissory note issued to GENBAND equity holders	22,500
Fair value of total consideration	$	479,433

Fair value of assets acquired and liabilities assumed:
Current assets, net of cash acquired	$	99,126
Property and equipment	16,770
Intangible assets:
In-process research and development	5,600
Developed technology	129,000
Customer relationships	101,300
Trade names	900
Goodwill	285,825
Other noncurrent assets	6,732
Revolving credit facility	(17,930		)
Deferred revenue	(32,390		)
Other current liabilities	(80,023		)
Deferred revenue, net of current	(6,804		)
Other long-term liabilities	(28,673		)
	$	479,433

The valuation of the acquired intangible assets is inherently subjective and relies on significant unobservable inputs. The Company used an income approach to value the acquired developed technology, customer relationships and trade name intangible assets. The valuation for each of these intangible assets was based on estimated projections of expected cash flows to be generated by the assets, discounted to the present value at discount rates commensurate with perceived risk. The valuation assumptions take into consideration the Company's estimates of customer attrition, technology obsolescence and revenue growth projections. The Company will reclassify its in-process research and development intangible assets to developed technology intangible assets in the periods that the related products became generally available and begin to record amortization expense for such developed technology intangible assets at that time. The Company is amortizing the identifiable intangible assets in relation to the expected cash flows from the individual intangible assets over their respective useful lives, which have a weighted average life of 8.3 years (see Note 9). Goodwill resulting from the transaction is primarily due to expected synergies between the combined companies and is not deductible for tax purposes.

Pro Forma Results

The following unaudited pro forma information presents the condensed combined results of operations of Sonus and GENBAND for the years ended December 31, 2017 and 2016 as if the Merger had been completed on January 1, 2016, with adjustments to give effect to pro forma events that are directly attributable to the Merger. These pro forma adjustments include a reduction of historical GENBAND revenue for the fair value adjustment related to acquired deferred revenue, an increase in amortization expense for the acquired identifiable intangible assets, a decrease in historical GENBAND interest expense reflecting the extinguishment of certain of GENBAND's debt as a result of the Merger, net of the interest expense recorded in connection with the promissory note issued to certain GENBAND equity holders as part of the purchase consideration and the elimination of revenue and costs related to sales transactions between Sonus and GENBAND. Pro forma adjustments also include the elimination of acquisition- and integration-related costs directly attributable to the acquisition and incremental stock-based compensation expense directly attributable to the acquisition from the year ended December 31, 2017 and inclusion of such costs in the year ended December 31, 2016.

The unaudited pro forma results do not reflect any operating efficiencies or potential cost savings that may result from the consolidation of the operations of Sonus and GENBAND. Accordingly, these unaudited pro forma results are presented for illustrative purposes and are not intended to represent or be indicative of the actual results of operations of the combined company that would have been achieved had the Merger occurred at the beginning of the periods presented, nor are they intended to represent or be indicative of future results of operations (in thousands, except per share amounts):



	Year ended December 31,
	2017			2016
	(unaudited)
Revenue	$	615,286		$	631,914
Net loss	$	(69,741	)	$	(147,394	)
Loss per share	$	(0.69	)	$	(1.46	)

Taqua, LLC

The Company acquired Taqua, a privately-held company, on September 26, 2016 (the "Taqua Acquisition Date"). Taqua enables the transformation of software-based service provider networks to deliver next-generation voice, video and messaging services, including VoIP, VoWiFi and VoLTE. In consideration for the acquisition of Taqua, Sonus paid $19.9 million in cash to the sellers on the Taqua Acquisition Date, net of cash acquired. The Company also entered into an Earn-Out Agreement, dated as of September 26, 2016, with Taqua Holdings, LLC and Jeffrey L. Brawner, the seller representative in the transaction, under which there is the potential for additional cash payments of up to $65 million in the aggregate to the sellers if certain annual revenue thresholds are exceeded as measured annually through 2020. The Company had initially recorded $10 million of contingent consideration as of the Taqua Acquisition Date, with the estimate based on historical sales and probability weighted cash flows related to forecasted sales. Because there are unobservable inputs to the valuation methodology that are significant to the measurement of its fair value, namely, forecasted sales, the Company had categorized the earn-out at Level 3 within the fair value hierarchy. During the fourth quarter of 2016, the Company reassessed the historical and updated forecasted sales and accordingly, reversed the previous estimated contingent consideration such that as of December 31, 2016, no incremental contingent consideration was recorded. The Company regularly reassesses the historical and updated forecasted sales and will adjust the amount recorded for estimated contingent consideration as required. As of December 31, 2017, no incremental contingent consideration was recorded.

The transaction has been accounted for as a business combination and the financial results of Taqua have been included in the Company's consolidated financial statements for the period subsequent to its acquisition.

The Company finalized its valuation of the identifiable intangible assets in the second quarter of 2017. During the first quarter of 2017 and the fourth quarter of 2016, the Company recorded changes to the initial preliminary purchase price allocation. The primary adjustments in the first quarter of 2017 were a $0.4 million increase to current liabilities and a $0.1 million increase to noncurrent liabilities. During the fourth quarter of 2016, the Company recorded changes to the initial preliminary purchase price allocation. The primary adjustments recorded in the fourth quarter of 2016 were the aforementioned reversal of the $10 million of previously recorded contingent consideration, a reduction of $12.1 million to the developed technology intangible asset and an increase of $5.5 million to the customer relationship intangible asset. These adjustments, as well as other immaterial adjustments to other balance sheet accounts, resulted in a net reduction to goodwill of $2.2 million. Based on this final purchase price allocation, the Company recorded $9.6 million of goodwill, which is primarily due to expected synergies between the combined companies and expanded market opportunities resulting from the expanded product offering portfolio. The goodwill is deductible for tax purposes.

A summary of the final allocation of the purchase consideration for Taqua is as follows (in thousands):



Fair value of consideration transferred:
Cash, net of cash acquired	$	19,919

Fair value of assets acquired and liabilities assumed:
Current assets	$	3,347
Property and equipment	1,478
Intangible assets:
Developed technology	2,100
Customer relationships	9,510
Goodwill	9,581
Other noncurrent assets	23
Current liabilities	(5,435		)
Long-term liabilities	(685		)
	$	19,919

The Company's revenue for the year ended December 31, 2016 included $1.9 million of revenue attributable to Taqua since the Taqua Acquisition Date. The inclusion of Taqua's operations for the period from the Taqua Acquisition Date to December 31, 2016 in the Company's financial results for the year ended December 31, 2016 increased the Company's loss by $4.7 million. The Company has not provided pro forma financial information, as the historical amounts were not significant to the Company's consolidated financial statements.

SDN Business of Treq Labs, Inc.

On January 2, 2015 (the "Treq Asset Acquisition Date"), the Company acquired from Treq Labs, Inc. ("Treq"), certain assets related to Treq's business of designing, developing, marketing, selling, servicing and maintaining SDN technology, SDN controller software and SDN management software (the "SDN Business"). The SDN Business provides solutions that optimize networks for voice, video and UC for both enterprise and service provider customers. In consideration for the acquisition of the SDN Business, Sonus paid $10.1 million in cash on the Treq Asset Acquisition Date, and an additional consideration payment of $750,000 on each of July 2, 2015 and January 4, 2016. The Company also entered into an Earn-Out Agreement, dated as of January 2, 2015, with Treq and Karl F. May, the seller representative in the transaction (the "Earn-Out Agreement"), under which the Company agreed to issue up to an aggregate of 1.3 million shares of common stock over a three-year period subsequent to the Treq Asset Acquisition Date if aggregate revenue thresholds of at least $60 million are achieved by the SDN Business during that period, and up to an aggregate of an additional 2.2 million shares of common stock (3.5 million shares in total) if aggregate revenue thresholds of at least $150 million are achieved by the SDN Business during that period. If the initial revenue thresholds are not met, no shares will be issued. Based on historical and forecasted sales, no incremental contingent consideration was recorded either initially as of the Treq Asset Acquisition Date or through December 31, 2017. Any shares issued pursuant to the Earn-Out Agreement will be issued in reliance on the exemption from registration available under Section 4(a)(2) of the Securities Act of 1933, as amended (the "Securities Act"), and will be subsequently registered for resale under the Securities Act by the Company.

The transaction has been accounted for as a business combination. The Company finalized its valuation of the identifiable intangible assets in the second quarter of fiscal 2015. Based on the purchase price allocation, the Company recorded $1.0 million of goodwill, primarily due to expected synergies between the combined companies and expanded market opportunities. The goodwill is deductible for tax purposes.

A summary of the final allocation of the purchase consideration for the SDN Business is as follows (in thousands):



Fair value of consideration transferred:
Cash, net of cash acquired	$	11,647
Fair value of assets acquired:
Intangible assets:
In-process research and development	$	9,100
Developed technology	1,500
Goodwill	1,047
	$	11,647

The valuation of the acquired intangible assets is inherently subjective and relies on significant unobservable inputs. The Company used an income approach to value the acquired in-process research and development and developed technology intangible assets. The valuation for each of these intangible assets was based on estimated projections of expected cash flows to be generated by the assets, discounted to the present value at discount rates commensurate with perceived risk. The valuation assumptions take into consideration the Company's estimates of technology attrition and revenue growth projections. In the three months ended September 25, 2015, the Company reclassified $7.5 million of its in-process research and development intangible assets to its developed technology intangible assets. During the three months ended March 31, 2016, the Company reclassified the remaining $1.6 million of in-process research and development intangible assets to its developed technology intangible assets. These amounts were reclassified from in-process research and development intangible assets to developed technology intangible assets in the periods that the related products became generally available and began to record amortization expense for such developed technology intangible assets. The Company is amortizing the identifiable intangible assets in relation to the expected cash flows from the individual intangible assets over their respective useful lives (see Note 9).

The Company has not disclosed the amount of revenue or earnings of the SDN Business since the SDN Business Acquisition Date or pro forma financial information, as these amounts are not significant to the Company's consolidated financial statements.

Acquisition- and Integration-Related Expenses

Acquisition- and integration-related expenses include those expenses related to acquisitions that would otherwise not have been incurred by the Company. The acquisition-related expenses include professional and services fees, such as legal, audit, consulting, paying agent and other fees, and expenses related to cash payments to certain former executives of the acquired businesses in connection with their employment agreements. The integration-related expenses represent incremental costs related to combining the two companies, such as third-party consulting and other third-party services related to merging the two separate companies' systems and processes. The amount recorded in the year ended December 31, 2017 primarily relates to the Merger, with a nominal amount related to the acquisition of Taqua. The amount recorded in the year ended December 31, 2016 relates to professional fees in connection with the acquisition of Taqua. The amount recorded in the year ended December 31, 2015 relates to professional fees in connection with the acquisition of the SDN Business.

The components of acquisition-related costs incurred in the years ended December 31, 2017, 2016 and 2015 are as follows (in thousands):



	Year ended December 31,
	2017		2016		2015
Professional and services fees (acquisition-related)	$	11,916	$	1,152	$	131
Management bonuses (acquisition-related)	931		—		—
Integration-related expenses	1,916		—		—
	$	14,763	$	1,152	$	131

EARNINGS (LOSS) PER SHARE

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

EARNINGS (LOSS) PER SHARE

Basic earnings (loss) per share is computed by dividing net income (loss) by the weighted average number of shares outstanding during the period. For periods in which the Company reports net income, diluted net income per share is determined by using the weighted average number of common and dilutive common equivalent shares outstanding during the period unless the effect is antidilutive.

The calculations of shares used to compute basic and diluted loss per share are as follows (in thousands):



	Year ended December 31,
	2017	2016	2015
Weighted average shares outstanding—basic	58,822	49,385	49,560
Potential dilutive common shares	—	—	—
Weighted average shares outstanding—diluted	58,822	49,385	49,560

Options to purchase the Company's common stock, unvested shares of restricted stock and unvested shares underlying performance-based stock grants aggregating 2.5 million shares for the year ended December 31, 2017 have not been included in the computation of diluted loss per share because their effect would have been antidilutive. Options to purchase the Company's common stock, unvested shares of restricted stock, unvested shares underlying performance-based stock grants and shares in connection with future purchases under the Company's Amended and Restated 2000 Employee Stock Purchase Plan, as amended (the "ESPP"), aggregating 8.0 million shares for the year ended December 31, 2016 and 8.2 million shares for the year ended December 31, 2015 have not been included in the computation of diluted loss per share because their effect would have been antidilutive.

CASH EQUIVALENTS AND INVESTMENTS

12 Months Ended

Dec. 31, 2017

Investments, All Other Investments [Abstract]

CASH EQUIVALENTS, MARKETABLE SECURITIES AND INVESTMENTS

CASH EQUIVALENTS AND INVESTMENTS

The Company invests in debt and equity instruments, primarily U.S. government-backed, municipal and corporate obligations, which management believes to be high quality (investment grade) credit instruments.

During the year ended December 31, 2017, the Company sold $51.6 million of its available-for-sale securities, primarily to provide the cash consideration and other acquisition-related payments in connection with the Merger. During the year ended December 31, 2016, the Company sold $4.9 million of its available-for-sale securities. The Company recognized nominal gross gains and losses from the sales of these securities. The Company did not sell any of its available-for-sale securities during the year ended December 31, 2015.

Investments with continuous unrealized losses for one year or greater at December 31, 2017 were nominal; however, since the Company does not intend to sell these securities and does not believe it will be required to sell any securities before they recover in value, it does not believe these declines are other-than-temporary.

On a quarterly basis, the Company reviews its investments to determine if there have been any events that could create a credit impairment. Based on its reviews, the Company does not believe that any impairment existed with its current holdings at December 31, 2017.

The amortized cost, gross unrealized gains and losses and fair value of the Company's cash equivalents and investments at December 31, 2017 and 2016 were comprised of the following (in thousands):



	December 31, 2017
	Amortized cost		Unrealized gains		Unrealized losses			Fair value
Cash equivalents	$	1,254	$	—	$	—		$	1,254

Short-term investments
U.S. government agency notes	$	4,091	$	—	$	(19	)	$	4,072
Corporate debt securities	8,048		—		(31		)	8,017
Certificates of deposit	5,135		—		—			5,135
	$	17,274	$	—	$	(50	)	$	17,224
Investments
U.S. government agency notes	$	3,992	$	—	$	(28	)	$	3,964
Corporate debt securities	3,908		—		(24		)	3,884
Certificates of deposit	1,183		—		—			1,183
	$	9,083	$	—	$	(52	)	$	9,031



	December 31, 2016
	Amortized cost		Unrealized gains		Unrealized losses			Fair value
Cash equivalents	$	6,619	$	—	$	—		$	6,619

Short-term investments
Municipal obligations	$	3,264	$	—	$	(3	)	$	3,261
U.S. government agency notes	16,477		3		(3		)	16,477
Corporate debt securities	41,893		4		(45		)	41,852
Certificates of deposit	246		—		—			246
	$	61,880	$	7	$	(51	)	$	61,836
Investments
U.S. government agency notes	$	19,473	$	3	$	(39	)	$	19,437
Corporate debt securities	10,520		—		(44		)	10,476
Certificates of deposit	2,458		—		—			2,458
	$	32,451	$	3	$	(83	)	$	32,371

The Company's available-for-sale debt securities that are classified as Investments in the consolidated balance sheet mature after one year but within two years or less from the balance sheet date.

Fair Value Hierarchy

Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or a liability. The three-tier fair value hierarchy is based on the level of independent, objective evidence surrounding the inputs used to measure fair value. A financial instrument's categorization within the fair value hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The fair value hierarchy is as follows:

Level 1. Level 1 applies to assets or liabilities for which there are quoted prices in active markets for identical assets or liabilities.

Level 2. Level 2 applies to assets or liabilities for which there are inputs that are directly or indirectly observable in the marketplace, such as quoted prices for similar assets or liabilities in active markets or quoted prices for identical assets or liabilities in markets with insufficient volume or infrequent transactions (less active markets).

Level 3. Level 3 applies to assets or liabilities for which there are unobservable inputs to the valuation methodology that are significant to the measurement of the fair value of the assets or liabilities.

The following table shows the fair value of the Company's financial assets at December 31, 2017 and 2016. These financial assets are comprised of the Company's available-for-sale debt securities and reported under the captions Cash and cash equivalents, Short-term investments and Investments in the consolidated balance sheets (in thousands):



			Fair value measurements at December 31, 2017 using:
	Total carrying value at December 31, 2017		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
Cash equivalents	$	1,254	$	1,254	$	—	$	—

Short-term investments
U.S. government agency notes	$	4,072	$	—	$	4,072	$	—
Corporate debt securities	8,017		—		8,017		—
Certificates of deposit	5,135		—		5,135		—
	$	17,224	$	—	$	17,224	$	—
Investments
U.S. government agency notes	$	3,964	$	—	$	3,964	$	—
Corporate debt securities	3,884		—		3,884		—
Certificates of deposit	1,183		—		1,183		—
	$	9,031	$	—	$	9,031	$	—



			Fair value measurements at December 31, 2016 using:
	Total carrying value at December 31, 2016		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
Cash equivalents	$	6,619	$	6,619	$	—	$	—

Short-term investments
Municipal obligations	$	3,261	$	—	$	3,261	$	—
U.S. government agency notes	16,477		—		16,477		—
Corporate debt securities	41,852		—		41,852		—
Certificates of deposit	246		—		246		—
	$	61,836	$	—	$	61,836	$	—
Investments
U.S. government agency notes	$	19,437	$	—	$	19,437	$	—
Corporate debt securities	10,476		—		10,476		—
Certificates of deposit	2,458		—		2,458		—
	$	32,371	$	—	$	32,371	$	—

The Company's marketable securities and investments have been valued with the assistance of valuations provided by third-party pricing services, as derived from such services' pricing models. Inputs to the models may include, but are not limited to, reported trades, executable bid and asked prices, broker/dealer quotations, prices or yields of securities with similar characteristics, benchmark curves or information pertaining to the issuer, as well as industry and economic events. The pricing services may use a matrix approach, which considers information regarding securities with similar characteristics to determine the valuation for a security. The Company is ultimately responsible for the consolidated financial statements and underlying estimates. Accordingly, the Company assesses the reasonableness of the valuations provided by the third-party pricing services by reviewing actual trade data, broker/dealer quotes and other similar data, which are obtained from quoted market prices or other sources.

ACCOUNTS RECEIVABLE, NET

12 Months Ended

Dec. 31, 2017

Receivables [Abstract]

ACCOUNTS RECEIVABLE, NET

Accounts receivable, net, consisted of the following (in thousands):



	December 31,
	2017			2016
Accounts receivable	$	165,229		$	53,872
Allowance for doubtful accounts	(73		)	(10		)
Accounts receivable, net	$	165,156		$	53,862

The Company's allowance for doubtful accounts activity was as follows (in thousands):



Year ended December 31,	Balance at beginning of year		Charges to expense		Charges (credits) to other accounts (deferred revenue)			Write-offs			Balance at end of year
2017	$	10	$	154	$	(56	)	$	(35	)	$	73
2016	$	10	$	10	$	—		$	(10	)	$	10
2015	$	58	$	17	$	—		$	(65	)	$	10

- Definition

The entire disclosure for claims held for amounts due a entity, excluding financing receivables. Examples include, but are not limited to, trade accounts receivables, notes receivables, loans receivables. Includes disclosure for allowance for credit losses.

+ References + Details

PROPERTY AND EQUIPMENT

12 Months Ended

Dec. 31, 2017

Property, Plant and Equipment [Abstract]

PROPERTY AND EQUIPMENT

Property and equipment consisted of the following (in thousands):



		December 31,
	Useful Life	2017			2016
Equipment	2-5 years	$	67,415		$	63,622
Software	2-5 years	21,977			19,378
Furniture and fixtures	3-5 years	1,892			698
Leasehold improvements	Shorter of the life of the lease or estimated useful life (1-5 years)	18,428			11,757
		109,712			95,455
Less accumulated depreciation and amortization		(84,932		)	(83,714		)
Property and equipment, net		$	24,780		$	11,741

The Company recorded depreciation and amortization expense related to property and equipment of $8.5 million for the year ended December 31, 2017, $8.0 million for the year ended December 31, 2016 and $12.0 million for the year ended December 31, 2015. During each of the years ended December 31, 2017 and 2016, the Company disposed of certain property and equipment that was fully depreciated at the time of disposal, which resulted in reductions in both Cost and Accumulated depreciation.

Property and equipment under capital leases included in the amounts above were as follows (in thousands):



	December 31,
	2017			2016
Cost	$	664		$	173
Less accumulated depreciation	(180		)	(68		)
Property and equipment under capital leases, net	$	484		$	105

The net book values of the Company's property and equipment by geographic area were as follows (in thousands):



	December 31,
	2017		2016
United States	$	17,576	$	7,939
Canada	1,740		246
Asia/Pacific	3,853		2,963
Europe	1,400		593
Other	211		—
	$	24,780	$	11,741

INTANGIBLE ASSETS AND GOODWILL

12 Months Ended

Dec. 31, 2017

Goodwill and Intangible Assets Disclosure [Abstract]

INTANGIBLE ASSETS AND GOODWILL

The Company's intangible assets at December 31, 2017 and 2016 consisted of the following (in thousands):



December 31, 2017	Weighted average amortization period (years)	Cost		Accumulated amortization		Net carrying value
In-process research and development	*	$	5,600	$	—	$	5,600
Developed technology	6.90	153,380		24,211		129,169
Customer relationships	9.32	120,840		12,015		108,825
Trade names	3.00	900		80		820
Internal use software	3.00	730		730		—
	7.77	$	281,450	$	37,036	$	244,414



December 31, 2016	Weighted average amortization period (years)	Cost		Accumulated amortization		Net carrying value
Developed technology	6.54	$	34,980	$	16,453	$	18,527
Customer relationships	5.78	19,540		7,870		11,670
Internal use software	3.00	730		730		—
	6.23	$	55,250	$	25,053	$	30,197

* An in-process research and development intangible asset has an indefinite life until the product is generally available, at which time such asset is typically reclassified to developed technology.

Amortization expense for intangible assets for the years ended December 31, 2017, 2016 and 2015 was as follows (in thousands):



	Year ended December 31,						Statement of operations classification
	2017		2016		2015		Statement of operations classification
Developed technology	$	18,358	$	6,038	$	5,222	Cost of revenue - product
Customer relationships	4,145		1,462		1,723		Sales and marketing
Trade names	80		—		—		Sales and marketing
Internal use software	—		—		162		Cost of revenue - product
	$	22,583	$	7,500	$	7,107

In connection with the preparation of its financial statements for the fourth quarter of 2017, the Company reviewed its intangible assets and other long-lived assets for impairment indicators. The Company determined that a triggering event had occurred relative to one of its developed technology intangible assets that had been previously acquired. During 2017, the Company discontinued its ongoing development of this technology and determined that there were no alternative uses of the technology within either its existing or future product lines. As a result, the Company recorded an impairment charge of $5.5 million to write down the carrying value of the asset to zero. This expense is included as a component of Cost of revenue - product in the Company's consolidated statements of operations for the year ended December 31, 2017.

Estimated future amortization expense for the Company's intangible assets at December 31, 2017 was as follows (in thousands):



Years ending December 31,
2018	$	46,013
2019	39,109
2020	38,590
2021	32,472
2022	26,534
Thereafter	61,696
	$	244,414

Goodwill is recorded when the consideration for an acquisition exceeds the fair value of net tangible and identifiable intangible assets acquired. The changes in the carrying value of the Company's goodwill in the years ended December 31, 2017 and 2016 were as follows (in thousands):



	Year ended December 31,
	2017			2016
Balance at January 1
Goodwill	$	52,499		$	43,416
Accumulated impairment losses	(3,106		)	(3,106		)
	49,393			40,310
Acquisition of GENBAND	285,825			—
Acquisition of Taqua and subsequent purchase accounting adjustments	498			9,083
Balance at December 31	$	335,716		$	49,393

The components of the Company's goodwill balances at December 31, 2017 and 2016 were as follows:



	December 31,
	2017			2016
Goodwill	$	338,822		$	52,499
Accumulated impairment losses	(3,106		)	(3,106		)
	$	335,716		$	49,393

- Definition

The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final. May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.

+ References + Details

RESTRUCTURING ACCRUAL

12 Months Ended

Dec. 31, 2017

Restructuring and Related Activities [Abstract]

RESTRUCTURING ACCRUAL

RESTRUCTURING ACCRUALS

The Company recorded restructuring expense aggregating $9.4 million in the year ended December 31, 2017, $2.7 million in the year ended December 31, 2016 and $2.1 million in the year ended December 31, 2015.

Merger Restructuring Initiative

In connection with the Merger, the Company's management approved a restructuring plan in the fourth quarter of 2017 to eliminate certain redundant positions and facilities within the combined companies (the "Merger Restructuring Initiative"). In connection with this initiative, the Company recorded $8.5 million of restructuring expense in 2017 for severance and related costs for approximately 120 employees. The Company expects to record additional restructuring expense in 2018 in connection with this initiative for redundant facilities and severance for approximately 40 additional employees as it continues to combine the two businesses and benefit from operational synergies. The Company expects the amount accrued at December 31, 2017 will be paid in 2018. The Company anticipates it will record additional future expense in connection with this initiative for headcount and redundant facilities aggregating approximately $12 million. The Company believes that the payments related to this expected additional future expense will be completed by early 2019.

A summary of the Merger Restructuring Initiative accrual activity for the year ended December 31, 2017 is as follows (in thousands):



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2017
Severance	$	—	$	8,508	$	—	$	(913	)	$	7,595

The Company assumed GENBAND's previously recorded restructuring liability, totaling $4.1 million, on the Merger Date (the "GENBAND Restructuring Initiative"). Of this amount, $3.7 million related to severance and related costs and $0.4 million related to facilities. The Company does not expect to record additional expense in connection with this initiative with the exception of adjustments for changes in estimated costs. The Company expects that the payments related to this assumed liability will be completed in 2018. A summary of the GENBAND Restructuring Initiative accrual activity for the year ended December 31, 2017 is as follows (in thousands):



Year ended December 31, 2017	Balance at January 1, 2017		Liability assumed in connection with Merger		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	—	$	3,663	$	—	$	(158	)	$	(1,589	)	$	1,916
Facilities	—		431		—		(123		)	(103		)	205
	$	—	$	4,094	$	—	$	(281	)	$	(1,692	)	$	2,121

2016 Restructuring Initiative

In July 2016, the Company announced a program (the "2016 Restructuring Initiative") to further accelerate its investment in new technologies, as the communications industry migrates to a cloud-based architecture and as the Company pursues new strategic initiatives, such as new products and an expanded go-to-market footprint in selected geographies and discrete vertical markets. The Company recorded $2.0 million of restructuring expense in the aggregate in connection with this initiative, comprised of $1.9 million for severance and related costs and $0.1 million to abandon its facility in Rochester, New York (the "Rochester Facility"). The actions under the 2016 Restructuring Initiative have been implemented and accordingly, the Company does not expect to record additional expense in connection with this initiative. The amounts accrued for severance and related costs had been fully paid by the end of the third quarter of 2017. The Company expects that the amounts accrued for facilities will be paid by the end of October 2019, when the lease on the Rochester Facility expires.

In connection with the 2016 Restructuring Initiative, the Company recorded $0.5 million of restructuring expense in the year ended December 31, 2017, including adjustments for changes in estimated costs, comprised of $0.4 million for severance and related costs and $0.1 million related to the Company's Rochester Facility. The Company recorded $1.5 million of expense in the year ended December 31, 2016 related to headcount reductions. Summaries of the 2016 Restructuring Initiative accrual activity for the years ended December 31, 2017 and 2016 are as follows (in thousands):



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	497	$	405	$	(26	)	$	(876	)	$	—
Facilities	—		126		—			(31		)	95
	$	497	$	531	$	(26	)	$	(907	)	$	95



Year ended December 31, 2016	Balance at January 1, 2016		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2016
Severance	$	—	$	1,484	$	—	$	(987	)	$	497

Taqua Restructuring Initiative

In connection with the acquisition of Taqua, the Company's management approved a restructuring plan in the third quarter of 2016 to eliminate certain redundant positions within the combined companies. On October 24, 2016, the Audit Committee of the Board of Directors of the Company approved a broader Taqua restructuring plan related to headcount and redundant facilities (both restructuring plans, the "Taqua Restructuring Initiative"). The Company recorded $1.8 million of restructuring expense in the aggregate in connection with this initiative, comprised of $1.2 million for severance and related costs and $0.6 million related to the elimination of redundant facilities, including adjustments recorded for changes in cost estimates for the planned restructuring activities. The actions under the Taqua Restructuring Initiative have been implemented and accordingly, the Company does not expect to record additional expense in connection with this initiative. The amounts accrued for severance and related costs had been fully paid by the end of the third quarter of 2017. The Company expects that the amounts accrued for facilities will be paid by the end of 2018.

In connection with the Taqua Restructuring Initiative, the Company recorded $0.7 million of restructuring expense, including adjustments for changes in estimated costs, in the year ended December 31, 2017, comprised of $0.2 million for severance and related costs and $0.5 million related to redundant facilities. The Company recorded $1.2 million of restructuring expense for this initiative in the year ended December 31, 2016, comprised of $1.0 million for severance and related costs and $0.2 million related to redundant facilities. Summaries of the Taqua Restructuring Initiative accrual activity for the years ended December 31, 2017 and 2016 are as follows (in thousands):



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	384	$	245	$	(49	)	$	(580	)	$	—
Facilities	218		508		—			(361		)	365
	$	602	$	753	$	(49	)	$	(941	)	$	365



Year ended December 31, 2016	Balance at January 1, 2016		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2016
Severance	$	—	$	971	$	—	$	(587	)	$	384
Facilities	—		218		—		—			218
	$	—	$	1,189	$	—	$	(587	)	$	602

2015 Restructuring Initiative

To better align the Company's cost structure to its then-current revenue expectations, in April 2015, the Company announced a cost reduction review. As part of this review, on April 16, 2015, the Company initiated a restructuring plan to reduce its workforce by approximately 150 positions, or 12.5% of its worldwide workforce (the "2015 Restructuring Initiative"). In connection with the 2015 Restructuring Initiative, the Company recorded $3.8 million of restructuring expense for severance and related costs in the year ended December 31, 2015. The Company recorded $67,000 in the year ended December 31, 2016 to adjust the amount expected to ultimately be paid for severance. Summaries of the 2015 Restructuring Initiative accrual for the years ended December 31, 2017 and 2016 are as follows (in thousands):



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2017
Severance	$	168	$	—	$	—	$	(168	)	$	—



Year ended December 31, 2016	Balance at January 1, 2016		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2016
Severance	$	749	$	—	$	67	$	(648	)	$	168

Balance Sheet Classification

The current portions of accrued restructuring are included as a component of Accrued expenses in the consolidated balance sheets. The long-term portions of accrued restructuring are included as a component of Other long-term liabilities in the consolidated balance sheets. The long-term portions of accrued restructuring were $0.2 million at December 31, 2017 and $0.1 million at December 31, 2016. These amounts represent future lease payments on restructured facilities.

DEBT	12 Months Ended
DEBT	Dec. 31, 2017
Debt Disclosure [Abstract]
DEBT	DEBT Assumed Senior Secured Credit Agreement On the Merger Date and in connection with the Merger, the Company assumed GENBAND's Senior Secured Credit Agreement with Silicon Valley Bank (the "Prior Credit Agreement"), which had outstanding borrowings and letters of credit totaling $17.9 million and $2.9 million, respectively, and an average interest rate of 4.67%. GENBAND had entered the Prior Credit Agreement with Silicon Valley Bank ("SVB") effective July 1, 2016, with two of its operating subsidiaries as borrowers and GENBAND as the guarantor. The Prior Credit Agreement had a maturity date of July 1, 2019 and provided for revolving loans, including letters of credit and swingline loans, not to exceed $50 million in total, with potential further increases of $75 million available for a total revolving line of credit of up to $125 million. Senior Secured Credit Facility On December 21, 2017, the Company entered into a Senior Secured Credit Facilities Credit Agreement (the “Credit Facility”), by and among the Company, as a guarantor, Sonus Networks, Inc., as the borrower (“Borrower”), Silicon Valley Bank, as administrative agent (in such capacity, the “Administrative Agent”), issuing lender, swingline lender and lead arranger and the lenders party thereto (each referred to individually as a “Lender”, and collectively, the “Lenders”), which refinanced the Prior Credit Agreement. The Credit Facility includes $100 million of commitments from the lenders to the Borrower, the full amount of which is available for revolving loans, a $15 million sublimit that is available for letters of credit and a $15 million sublimit that is available for swingline loans. The Credit Facility is scheduled to mature in December 2021, subject to a springing maturity if, on or before July 14, 2020, the existing promissory note issued to certain shareholders is not converted or extended to March 2022 or later. The Credit Facility includes procedures for additional financial institutions to become lenders, or for any existing lender to increase its commitment under the facility, subject to an available increase of $50 million for all incremental commitments under the Credit Facility. The indebtedness and other obligations under the Credit Facility are unconditionally guaranteed on a senior secured basis by the Company and GENBAND US LLC, a wholly-owned domestic subsidiary of the Company (collectively, the “Guarantors”) and each other material US domestic subsidiary of the Company. The Credit Facility is secured by first-priority liens on substantially all of the assets of the Borrower and the Guarantors, including the Company. The Credit Facility requires periodic interest payments until maturity. The Borrower may prepay all revolving loans under the Credit Facility at any time without premium or penalty (other than customary LIBOR breakage costs), subject to certain notice requirements. Revolving loans under the Credit Facility bear interest at the Borrower’s option at either the Eurodollar (LIBOR) rate plus a margin ranging from 2.50% to 3.00% per year or the base rate (the highest of the Federal Funds rate plus 0.50%, or the prime rate announced from time to time in The Wall Street Journal) plus a margin ranging from 1.50% to 2.00% per year (such margins being referred to as the “Applicable Margin”). The Applicable Margin varies depending on the Company’s consolidated leverage ratio (as defined in the Credit Facility). The base rate and the LIBOR rate are each subject to a zero percent floor. The Borrower is charged a commitment fee ranging from 0.25% to 0.40% per year on the daily amount of the unused portions of the commitments under the Credit Facility. Additionally, with respect to all letters of credit outstanding under the Credit Facility, the Borrower is charged a fronting fee of 0.125% per year and an outstanding letter of credit fee equal to the Applicable Margin for base rate loans ranging from 1.50% to 2.00% times the amount of the outstanding letters of credit. The Credit Facility includes financial covenants regarding its minimum consolidated quick ratio, minimum consolidated interest coverage ratio and maximum consolidated leverage ratio, all of which are defined in the Credit Facility and tested on a quarterly basis. In addition, the Credit Facility contains various covenants that, among other restrictions, limit the Company’s and its subsidiaries’ ability to enter into certain types of transactions, including, but not limited to: incurring or assuming indebtedness, making acquisitions or engaging in mergers, making investments, repurchasing equity and paying dividends, selling or otherwise transferring assets, changing the nature of its business and amending or making prepayments on certain junior debt. The Company was in compliance with all covenants of the Credit Facility as of December 31, 2017. The Credit Facility contains events of default that are customary for a secured credit facility. If an event of default relating to bankruptcy or other insolvency events with respect to a borrower occurs, all obligations under the Credit Facility will immediately become due and payable. If any other event of default exists under the Credit Facility, the lenders may accelerate the maturity of the obligations outstanding under the Credit Facility and exercise other rights and remedies, including charging a default rate of interest equal to 2.00% per year above the rate that would otherwise be applicable. In addition, if any event of default exists under the Credit Facility, the lenders may commence foreclosure or other actions against the collateral. If any default exists under the Credit Facility, or if the Borrower is unable to make any of the representations and warranties as stated in the Credit Facility at the applicable time, the Borrower will be unable to borrow funds or have letters of credit issued under the Credit Facility, which, depending on the circumstances prevailing at that time, could have a material adverse effect on the Borrower’s liquidity and working capital. At December 31, 2017, the Company had an outstanding debt balance of $20.0 million at an interest rate of 4.51% and $2.9 million of outstanding letters of credit at an interest rate of 2.00% under the Credit Facility. Promissory Note In connection with the Merger, on October 27, 2017, the Company issued a promissory note for $22.5 million to certain of GENBAND's equity holders (the "Promissory Note"). The Promissory Note does not amortize and the principal thereon is payable in full on the third anniversary of its execution. Interest on the Promissory Note is payable quarterly in arrears and accrues at a rate of 7.5% per year for the first six months after issuance, and thereafter at a rate of 10% per year. The failure to make any payment under the Promissory Note when due and, with respect to payment of any interest, the continuation of such failure for a period of thirty days thereafter, constitutes an event of default under the Promissory Note. If an event of default occurs under the Promissory Note, the payees may declare the entire balance of the Promissory Note due and payable (including principal and accrued and unpaid interest) within five business days of the payees' notification to the Company of such acceleration. Credit Agreement Sonus maintained a credit agreement by and among Sonus, as Borrower, Bank of America, N.A. ("Bank of America"), as Administrative Agent, Swing Line Lender and L/C Issuer, and the other lenders from time to time party thereto, entered into on June 27, 2014 (as amended, the "Sonus Credit Agreement"). The Sonus Credit Agreement expired by its terms on June 30, 2017 and was not renewed. Sonus did not have any amounts outstanding under the Sonus Credit Agreement at December 31, 2016.

COMMON STOCK REPURCHASES	12 Months Ended
COMMON STOCK REPURCHASES	Dec. 31, 2017
Equity [Abstract]
COMMON STOCK REPURCHASES AND UNDERWRITTEN OFFERING	COMMON STOCK REPURCHASES On July 29, 2013, Sonus announced that its Board of Directors had authorized a stock buyback program to repurchase up to $100 million of its common stock from time to time on the open market or in privately negotiated transactions. The timing and amount of any shares repurchased was determined by Sonus' management based on its evaluation of market conditions and other factors. The buyback program did not have a fixed expiration date but could be suspended or discontinued at any time. The buyback program was funded using Sonus' working capital. Ribbon did not assume the stock buyback program in connection with the Merger. During the year ended December 31, 2017, Sonus did not repurchase any shares under the stock buyback program. During the year ended December 31, 2016, Sonus spent $9.5 million, including transaction fees, to repurchase and retire 1.3 million shares of its common stock under the buyback program.

STOCK-BASED COMPENSATION PLANS

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

STOCK-BASED COMPENSATION PLANS

Amended and Restated Stock Incentive Plan

The Company's Amended and Restated Stock Incentive Plan (the "Plan"), provides for the award of options to purchase the Company's common stock ("stock options"), stock appreciation rights ("SARs"), restricted stock awards ("RSAs"), restricted stock units ("RSUs"), performance-based stock awards ("PSAs"), performance-based stock units ("PSUs") and other stock-based awards to employees, officers, directors (including those directors who are not employees or officers of the Company), consultants and advisors of the Company and its subsidiaries.

At its 2017 Annual Meeting of Stockholders held on June 9, 2017 (the "2017 Annual Meeting"), the Company's stockholders approved amendments to the Plan including, among other things, to increase the number of shares of the Company's common stock authorized for issuance under the Plan by 900,000 shares; provide that any dividends or dividend equivalents on applicable unvested equity grants will be paid to a participant only if and when such shares become free from the restrictions on transferability and forfeitability; explicitly require a participant who accepts an award under the Plan to be bound by any clawback policy that the Company has in effect or may adopt in the future; and eliminate the fungible ratio applied to awards of shares of restricted stock, restricted stock units , performance awards or other stock units (collectively, full value awards") granted on or after June 9, 2017, but maintained the fungible ratios applied to prior grants of full value awards such that full value awards that were granted under any prior ratio that applied at the time such awards were granted will continue to return to the Plan upon forfeiture of such awards at the previous applicable ratio.

In connection with the Merger, the Company assumed the Plan with all of its then-current terms and conditions.

At December 31, 2017, there were 5.4 million shares available for future issuance under the Plan.

2012 Stock Incentive Plan

In connection with the acquisition of PT, the Company assumed PT's 2012 Amended Performance Technologies, Incorporated Omnibus Incentive Plan, and subsequently renamed it the 2012 Stock Incentive Plan (the "2012 Plan"). In December 2014, all of the unissued shares under the 2012 Plan were transferred to the Plan. Any outstanding awards under the 2012 Plan that in the future expire, terminate, are canceled, surrendered or forfeited, or are repurchased by the Company will be returned to the Plan. Accordingly, at December 31, 2017, there were no shares available for future issuance under the 2012 Plan.

2008 Stock Incentive Plan

In connection with the acquisition of NET, the Company assumed NET's 2008 Equity Incentive Plan and subsequently renamed it the 2008 Stock Incentive Plan (the "2008 Plan"). In December 2014, all of the unissued shares under the 2008 Plan were transferred to the Plan. Any outstanding awards under the 2008 Plan that in the future expire, terminate, are canceled, surrendered or forfeited, or are repurchased by the Company will be returned to the Plan. Accordingly, at December 31, 2017, there were no shares available for future issuance under the 2008 Plan.

Treatment of Equity Awards in Connection with the Merger

In connection with the Merger, the Company accelerated the vesting of all outstanding stock options and certain outstanding full value awards. In addition, the vesting schedules of certain remaining unvested full value awards were adjusted. Such vesting and adjustments are described as follows:

Stock options - each stock option outstanding as of five business days prior to the Merger Date became vested in full as of that date (to the extent not previously vested), and the holders of such stock options were permitted to exercise their stock options from October 20, 2017 through October 24, 2017, after which date all remaining stock options, with certain exceptions, were canceled. The Company accelerated the vesting of 0.3 million stock options and subsequently canceled 4.5 million vested unexercised stock options in connection with this transaction. This activity is included in the stock options table below. Any stock options granted under the 2008 Plan and 2012 Plan were not canceled, as these plans do not permit such cancellations. These stock options will continue to be outstanding until they are either exercised or expire.

RSAs and RSUs - as prescribed by the Company's Plan, any unvested RSAs and RSUs that were scheduled to vest within one year from the Merger Date became vested in full as of the Merger Date. The vesting schedules of the remaining unvested RSAs and RSUs were then accelerated by one year. Certain executives had specific terms and conditions related to their RSAs detailed in their employment agreements or amendments thereto (the "employment terms"). The accelerated vesting of and future vesting schedule adjustments to the RSAs held by these individuals were completed in accordance with their individual employment terms. In accordance with the terms of their RSA grants, unvested RSAs held by the then-current members of the Board of Directors were accelerated on a pro rata basis based on the amount of time the unvested RSAs were outstanding compared to the originally scheduled vesting date. Unvested PSUs granted to the Company's former President and Chief Executive Officer, who separated from the Company effective December 13, 2017 (the "former CEO"), were converted to RSAs in accordance with his employment terms; certain of those converted grants were accelerated, and the remaining RSAs would continue to vest according to their terms, but with the elimination of any required satisfaction of the performance metrics associated with the awards when they were originally granted as PSUs. Accordingly, the release of the former CEO's RSAs that were converted from PSUs is included in the RSA table below. In total, the Company accelerated the vesting of and released 1.1 million RSAs and approximately 36,000 RSUs in connection with the Merger. The activity related to these RSAs and RSUs is included in the applicable tables below.

PSUs - any unvested PSUs were accelerated in accordance with the employment agreement of each individual PSU holder. The remaining unvested units will continue to vest according to their terms, with the exception of the PSU grants held by the former CEO, as discussed above. The Company accelerated the vesting of and released approximately 98,000 PSUs in connection with this transaction. The activity related to these PSUs is included in the PSU table below.

Executive Equity Arrangements

In 2015, the Company began to grant PSUs to certain of its executives. The terms of each PSU grant are such that up to one-third of the shares subject to the respective PSU grant will vest, if at all, on each of the respective first, second and third anniversaries of the date of grant, depending on the Company's total shareholder return ("TSR") compared to the TSR of the companies included in the Nasdaq Telecommunications Index for the same fiscal year, measured by the Compensation Committee after each of the fiscal years as defined by each grant (each, a "Performance Period"). The shares determined to be earned will vest on the anniversary of the grant date following each Performance Period. Shares subject to the PSUs that fail to be earned will be forfeited.

All of the PSUs include a market condition that required the use of a Monte Carlo simulation approach to model future stock price movements based upon the risk-free rate of return, the date of return, the volatility of each entity and the pair-wise covariance between each entity. These results were then used to calculate the grant date fair values of the respective PSUs. Because all of the PSUs granted have market conditions, the Company is required to record expense for the PSUs through their respective final vesting dates regardless of the number of shares that are ultimately earned.

On March 31, 2017, the Company granted an aggregate of 165,000 PSUs with both market and service conditions to five of its executives.

On April 1, 2016, the Company granted an aggregate of 131,250 PSUs with both market and service conditions to six of its executives (the "2016 PSUs"). In February 2017, the Compensation Committee determined that the performance metrics for the 2016 PSUs for the 2016 Performance Period had been achieved at the 90.4% level, and accordingly, 24,106 shares in the aggregate were released to the four executives holding such outstanding grants on March 16, 2017. The unearned shares relating to the 2016 Performance Period, aggregating 2,560 shares, were forfeited on March 16, 2017. These amounts are included in the PSU table below.

On March 16, 2015, the Company granted an aggregate of 131,250 PSUs with both market and service conditions to eight of its executives (the "2015 PSUs"). In February 2017, the Compensation Committee determined that the performance metrics for the 2015 PSUs for the 2016 Performance Period had been achieved at the 76.0% level, and accordingly, 23,750 shares in the aggregate were released to the four remaining executives holding such outstanding grants on April 1, 2017. The unearned shares relating to the 2016 Performance Period, aggregating 7,500 shares, were forfeited on April 1, 2017.

In connection with the Merger, as previously described above, PSUs held by the former CEO were converted to RSAs and certain other PSUs were accelerated and released in accordance with the individual employment terms of each PSU grantee. The vesting schedules of the remaining unvested PSUs were adjusted to continue to vest on their terms.

In connection with the Company's annual incentive program, 22 executives of the Company were given the choice to receive all or half of their fiscal year 2015 bonuses (the "2015 Bonus"), if any were earned, in the form of shares of the Company's common stock (the "2015 Bonus Shares"). Each executive could also elect not to participate in this program and to earn his or her 2015 Bonus, if any, in the form of cash. In September 2015, the Compensation Committee considered the impact on employee retention and incentive compensation caused by the drop in the price of the Company's common stock since January 2, 2015, and indicated its intent to pay all such executives their 2015 Bonus, if any was earned, in cash. As a result, at September 25, 2015, the Company reclassified the stock-based compensation expense recorded through that date in connection with the 2015 Bonus Shares aggregating $1.0 million from Additional paid-in capital to Accrued expenses and recorded incremental bonus expense of $1.3 million related to the estimated 2015 Bonus payment. The Company recorded bonus expense in the fourth quarter of 2015 and paid the cash bonuses in March 2016.

On December 14, 2017, the Company announced that, on and effective December 13, 2017, the Board of Directors appointed Franklin (Fritz) W. Hobbs as the Company's President and Chief Executive Officer, and the Former CEO resigned his position as the Company's President and Chief Executive Officer and as a member of the Board of Directors. In connection with the separation of the Former CEO from the Company and in accordance with his employment agreement with the Company, as amended, the Company accelerated the vesting of his then-unvested RSAs (including those that had been converted from PSUs in connection with the Merger, as described above). These shares are reported as "Vested" in the RSA table below. However, due to the terms of the Former CEO's separation agreement with the Company and applicable employment laws, the shares were not released until January 2018. Accordingly, these shares were not considered outstanding as of December 31, 2017.

Stock Options

Options are issued to purchase shares of common stock of the Company at prices that are equal to the fair market value of the shares on the date the option is granted. Options granted under the Stock Plan generally expire ten years from the date of grant. Outstanding options under the 2008 Plan generally expire seven years from the date of grant. Outstanding options under the 2012 Plan generally expire five years from the date of grant. The grant date fair value of options, adjusted for estimated forfeitures, is recognized as expense on a straight-line basis over the requisite service period, which is generally the vesting period. Forfeitures are estimated based on historical experience.

The activity related to the Company's outstanding stock options during the year ended December 31, 2017 was as follows:



	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (in thousands)
Outstanding at January 1, 2016	5,610,106		$	15.73
Granted	7,760		$	6.67
Exercised	(105,688	)	$	5.85
Forfeited	(35,907	)	$	13.82
Expired	(5,041,084	)	$	16.02
Outstanding at December 31, 2017	435,187		$	14.71	4.30	$	66
Vested or expected to vest at December 31, 2017	435,187		$	14.71	4.30	$	66
Exercisable at December 31, 2017	435,187		$	14.71	4.30	$	66

The grant date fair values of options to purchase common stock granted in the years ended December 31, 2017, 2016 and 2015 were estimated using the Black-Scholes valuation model with the following assumptions:



	Year ended December 31,
	2017	2016	2015
Risk-free interest rate	1.22% - 1.95%	1.00% - 1.61%	1.46%-1.75%
Expected dividends	—	—	—
Weighted average volatility	51.1%	54.8%	54.3%
Expected life (years)	5.0	5.0-10.0	5.0-6.0

The risk-free interest rate used is the average U.S. Treasury Constant Maturities Rate for the expected life of the award. The expected dividend yield of zero is based on the fact that the Company has never paid dividends and has no present intention to pay cash dividends. The expected life for stock options is based on a combination of the Company's historical option patterns and expectations of future employee actions.

The weighted average grant-date fair values of options granted during the year were $3.05 for the year ended December 31, 2017, $4.39 for the year ended December 31, 2016 and $7.30 for the year ended December 31, 2015.

The total intrinsic values of options exercised during the year were $0.2 million for the year ended December 31, 2017, $42,000 for the year ended December 31, 2016 and $0.9 million for the year ended December 31, 2015.

The Company received cash from option exercises of $0.6 million in the year ended December 31, 2017, $0.2 million in the year ended December 31, 2016 and $1.8 million in the year ended December 31, 2015.

Restricted Stock Grants - Restricted Stock Awards and Restricted Stock Units

The Company's outstanding restricted stock grants consist of both RSAs and RSUs. Holders of unvested RSAs have voting rights and rights to receive dividends, if declared; however, these rights are forfeited if the underlying unvested RSA shares are forfeited. Holders of unvested RSUs do not have such voting and dividend rights. The grant date fair value of restricted stock grants, adjusted for estimated forfeitures, is recognized as expense on a straight-line basis over the requisite service period. The fair value of restricted stock grants is determined based on the market value of the Company's shares on the date of grant.

The activity related to the Company's RSAs for the year ended December 31, 2017 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2017	2,030,028		$	9.69
Granted	1,763,912		$	6.77
Unvested PSUs converted to RSAs in connection with the Merger	95,834		$	9.44
Vested	(2,080,179	)	$	8.93
Forfeited	(113,013	)	$	8.80
Unvested balance at December 31, 2017	1,696,582		$	7.68

The activity related to the Company's RSUs for the year ended December 31, 2017 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2017	110,219		$	11.95
Granted	—		$	—
Vested	(80,374	)	$	7.06
Forfeited	(11,913	)	$	8.66
Unvested balance at December 31, 2017	17,932		$	6.99

The total fair value of vested restricted stock grant shares was $19.1 million in the year ended December 31, 2017, $10.1 million in the year ended December 31, 2016 and $8.5 million in the year ended December 31, 2015.

Performance-Based Stock Grants - Performance-Based Stock Units

In 2016, the Company's outstanding performance-based stock grants consisted of PSUs. Holders of unvested PSUs do not have voting and dividend rights. The Company recognizes the grant date fair value of PSUs on a graded attribution basis through the vest date of the respective awards so long as it remains probable that the related service conditions will be satisfied.

The activity related to the Company's PSUs for the year ended December 31, 2017 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2017	147,085		$	12.11
Granted	165,000		$	8.45
Unvested PSUs converted to RSAs in connection with the Merger	(95,834	)	$	9.44
Vested	(145,357	)	$	9.44
Forfeited	(10,060	)	$	11.87
Unvested balance at December 31, 2017	60,834		$	9.65

The total fair value of vested performance-based stock grant shares was $1.4 million in the year ended December 31, 2017, $0.2 million in the year ended December 31, 2016 and $0.6 million in the year ended December 31, 2015.

ESPP

The ESPP is designed to provide eligible employees of the Company and its participating subsidiaries an opportunity to purchase common stock of the Company through accumulated payroll deductions.

The ESPP provides for six-month consecutive offering periods, with the purchase price of the stock equal to 85% of the lesser of the market price on the first or last day of the offering period. The maximum number of shares of common stock an employee may purchase during each offering period is 500, subject to certain adjustments pursuant to the ESPP.

In May 2017, the Compensation Committee determined to suspend all offering periods under the ESPP, effective September 1, 2017, until such time after the Merger Date as the Compensation Committee determines is best in its sole discretion.

At December 31, 2017, 5.0 million shares, the maximum number of shares that may be issued under the ESPP, were authorized, and 1.4 million shares were available under the ESPP for future issuance.

Stock-Based Compensation

The consolidated statements of operations included stock-based compensation for the years ended December 31, 2017, 2016 and 2015 as follows (in thousands):



	Year ended December 31,
	2017		2016		2015
Product cost of revenue	$	514	$	359	$	317
Service cost of revenue	1,448		1,314		1,524
Research and development	7,337		5,014		5,439
Sales and marketing	4,885		6,209		5,423
General and administrative	11,473		6,872		8,996
	$	25,657	$	19,768	$	21,699

There was no income tax benefit for employee stock-based compensation expense for the years ended December 31, 2017, 2016 and 2015 due to the valuation allowance recorded.

Stock-based compensation expense recorded for the year ended December 31, 2017 included $8.6 million of incremental expense related to the acceleration of stock options and full value awards and subsequent adjustments to the vesting schedules of the remaining unvested full value awards in connection with the Merger. In addition, the Company recorded $1.6 million of incremental expense related to the accelerated vesting of RSAs held by the Former CEO in connection with his separation from the Company effective December 13, 2017. These incremental amounts were all recorded in the fourth quarter of 2017.

During the three months ended March 31, 2017, the Company reversed $1.0 million of incremental expense to correct an error in 2016 related to the acceleration of certain stock awards held by an executive who separated from the Company in 2016. Management had reviewed and considered the impact of the error and determined that it was not material to the Company's consolidated financial results for the third and fourth quarters of 2016, as well as the 2016 fiscal year. Management has also determined that the correction of this error is not material to the results of operations for the 2017 reporting periods.

At December 31, 2017, there was $7.5 million, net of expected forfeitures, of unrecognized stock-based compensation expense related to unvested stock options, RSAs, RSUs and PSUs. This expense is expected to be recognized over a weighted average period of approximately two years.

Common Stock Reserved

Common stock reserved for future issuance at December 31, 2017 consists of the following:



Amended and Restated Stock Incentive Plan	5,359,243
ESPP	1,431,513
	6,790,756

The Company's policy is to issue authorized but unissued shares upon the exercise of stock options, grant restricted common stock awards and settlement of restricted stock units and performance-based stock units, and authorize the purchase of shares of the Company's common stock under the ESPP.

EMPLOYEE DEFINED CONTRIBUTION PLANS

12 Months Ended

Dec. 31, 2017

Retirement Benefits [Abstract]

EMPLOYEE DEFINED CONTRIBUTION PLAN

EMPLOYEE DEFINED CONTRIBUTION PLANS

The Company offers 401(k) savings plans to eligible employees. In the year ended December 31, 2015, the Company did not provide a matching contribution for deferral contributions made by employees and accordingly, did not record expense related to its employee defined contribution plan. However, in June 2016, at the recommendation of the Compensation Committee, the Company’s Board of Directors elected to reinstate a discretionary limited 401(k) match program of up to $2,000 per year ($1,000 per each half-year) per eligible employee, contingent upon the Company’s achievement of certain financial metric targets set by the Compensation Committee. The matching contribution became effective July 1, 2016.

The Company assumed GENBAND's 401(k) savings plan in connection with the Merger.

The Company recorded $1.4 million of expense in the year ended December 31, 2017 and $0.6 million of expense related to its employee defined contribution plans in the year ended December 31, 2016. Effective January 1, 2018, the Company will match 50% of each employee's contributions to the 401(k) program up to 4% of the employee's eligible earnings, for a maximum match of 2% of eligible earnings.

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS

In connection with the Merger, the Company assumed GENBAND's defined benefit retirement plans that cover some employees at various international locations. The Company has also adopted GENBAND's policy to contribute amounts at least sufficient to satisfy the minimum amount required by applicable law and regulations, or to directly pay benefits where appropriate. Benefits under the defined benefit plans are typically based either on years of service and the employee's compensation (generally during a fixed number of years immediately before retirement) or on annual credits. The range of assumptions that are used for the non-U.S. defined benefit plans reflect the different economic environments within the various countries.

A reconciliation of the changes in the benefit obligations and fair value of the assets of the defined benefit plans for the period from the Merger Date to December 31, 2017, the funded status of the plans and the amounts recognized in the consolidated balance sheet as of December 31, 2017 are as follows (in thousands):



Changes in projected benefit obligations:
Projected benefit obligation at Merger Date	$	10,515
Service cost	68
Interest cost	25
Participant contributions	5
Benefits paid	(3		)
Net actuarial loss on obligation	562
Currency loss	312
Projected benefit obligation at December 31, 2017	$	11,484

Changes in plan assets:
Fair value of plan assets at Merger Date	$	3,776
Actual return on plan assets	(8		)
Employer contributions	22
Participant contributions	5
Additional charges	(4		)
Benefits paid	(3		)
Currency gain	105
Fair value of plan assets at December 31, 2017	$	3,893

Funded status at December 31, 2017	$	(7,591	)

Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	$	578

Amounts recognized in the consolidated balance sheet consist of:
Accrued compensation and benefits (current pension liability)	$	(67	)
Other long-term liabilities (non-current pension liability)	(7,524		)
Net amount recognized	$	(7,591	)

Plans with underfunded or non-funded accumulated benefit obligations at December 31, 2017 are as follows (in thousands):



Aggregate projected benefit obligation	$	11,484
Aggregate accumulated benefit obligation	$	7,793
Aggregate fair value of plan assets	$	3,893

Net periodic benefit costs for the period from the Merger Date to December 31, 2017 are as follows (in thousands):



Service cost	$	68
Interest cost	25
Expected return on plan assets	(8		)
Additional charges	4
Net periodic benefit costs	$	89

The Company made benefit payments of $3,000 for the period from the Merger Date to December 31, 2017. Expected benefit payments for the next ten years are as follows:



Years ending December 31,
2018	$	67
2019	68
2020	49
2021	50
2022	50
2023 to 2027	847
	$	1,131

The change in plan assets and benefit obligations recognized in other comprehensive loss before tax for the period from the Merger Date to December 31, 2017 was as follows (in thousands):



Net loss	$	578

The Company defers all actuarial gains and losses resulting from variances between actual results and economic estimates or actuarial assumptions. The unrecognized actuarial gains and losses are recorded as unrealized pension actuarial gains (losses) in accumulated other comprehensive loss. These unrecognized gains and losses are amortized as a component of net periodic benefit cost when the net gains and losses exceed 10% of the greater of the market value of plan assets or the projected benefit obligation at the beginning of the year. No amortization of accumulated other comprehensive loss, net, into net periodic benefit cost is expected for the year ended December 31, 2018.

The principal weighted average assumptions used to determine the benefit obligation at December 31, 2017 are as follows:



Discount rate	1.31	%
Rate of compensation increase	3.38	%

The principal weighted average assumptions used to determine net period benefit cost for the period from the Merger Date to December 31, 2017 are as follows:



Discount rate	1.49	%
Expected long-term return on plan assets	1.23	%
Rate of compensation increase	3.38	%

Assumed discount rates are used in the measurement of the projected, accumulated and vested benefit obligations and the service and interest components of net periodic pension cost. Estimated discount rates reflect the rates at which the pension benefits could be effectively settled. The Company determines the discount rates of the plans in the Euro zone using the iBoxx Euro Corporate AA Bond indices with appropriate adjustments for the duration of the plan obligations. In other international locations, the Company determines the discount rates based primarily on local AA to AAA rated Corporate Bond Indices and the Citigroup Pension Discount Curve with the duration appropriate to the duration of the plan obligations.

The plans in the Netherlands and Switzerland are funded through insurance contracts, which provide guaranteed interest credit. The fair value of the contract is derived from the insurance company's assessment of the minimum value of the benefits provided by the insurance contract. The methodology used to value the plan assets assumes that the value of the plan assets equals the guaranteed insured benefits. For consistency, the same discount rate used in the valuation of the benefit obligations is used to place a value on the plan assets. The assets are assumed to grow each year in line with the discount rate, and therefore, the expected return on the assets is set equal to the discount rate. The fair value of the combined plan assets was $3.9 million at December 31, 2017. The Company classifies the fair value of these plan assets as Level 2 in the fair value hierarchy as discussed in Note 5.

During the period from the Merger Date to December 31, 2017, employees in the Netherlands and Switzerland made contributions to the respective pension plans aggregating $5,000. Employee contributions to these plans are based on a fixed 5% of the relevant pensionable earnings. The Company funds these plans by contributing at least the minimum amount required by applicable regulations and as recommended by an independent actuary. During the period from October 27, 2017 to December 31, 2017, the Company contributed $22,000 to its pension plans and expects to contribute approximately $0.2 million to its pension plans in 2018.

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS

12 Months Ended

Dec. 31, 2017

Retirement Benefits [Abstract]

Pension Disclosure

EMPLOYEE DEFINED CONTRIBUTION PLANS

The Company assumed GENBAND's 401(k) savings plan in connection with the Merger.

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS



Changes in projected benefit obligations:
Projected benefit obligation at Merger Date	$	10,515
Service cost	68
Interest cost	25
Participant contributions	5
Benefits paid	(3		)
Net actuarial loss on obligation	562
Currency loss	312
Projected benefit obligation at December 31, 2017	$	11,484

Changes in plan assets:
Fair value of plan assets at Merger Date	$	3,776
Actual return on plan assets	(8		)
Employer contributions	22
Participant contributions	5
Additional charges	(4		)
Benefits paid	(3		)
Currency gain	105
Fair value of plan assets at December 31, 2017	$	3,893

Funded status at December 31, 2017	$	(7,591	)

Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	$	578

Amounts recognized in the consolidated balance sheet consist of:
Accrued compensation and benefits (current pension liability)	$	(67	)
Other long-term liabilities (non-current pension liability)	(7,524		)
Net amount recognized	$	(7,591	)

Plans with underfunded or non-funded accumulated benefit obligations at December 31, 2017 are as follows (in thousands):



Aggregate projected benefit obligation	$	11,484
Aggregate accumulated benefit obligation	$	7,793
Aggregate fair value of plan assets	$	3,893

Net periodic benefit costs for the period from the Merger Date to December 31, 2017 are as follows (in thousands):



Service cost	$	68
Interest cost	25
Expected return on plan assets	(8		)
Additional charges	4
Net periodic benefit costs	$	89

The Company made benefit payments of $3,000 for the period from the Merger Date to December 31, 2017. Expected benefit payments for the next ten years are as follows:



Years ending December 31,
2018	$	67
2019	68
2020	49
2021	50
2022	50
2023 to 2027	847
	$	1,131

The change in plan assets and benefit obligations recognized in other comprehensive loss before tax for the period from the Merger Date to December 31, 2017 was as follows (in thousands):



Net loss	$	578

The principal weighted average assumptions used to determine the benefit obligation at December 31, 2017 are as follows:



Discount rate	1.31	%
Rate of compensation increase	3.38	%

The principal weighted average assumptions used to determine net period benefit cost for the period from the Merger Date to December 31, 2017 are as follows:



Discount rate	1.49	%
Expected long-term return on plan assets	1.23	%
Rate of compensation increase	3.38	%

INCOME TAXES

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

INCOME TAXES

The components of loss from continuing operations before income taxes consisted of the following (in thousands):



	Year ended December 31,
	2017			2016			2015
Income (loss) before income taxes:
United States	$	(55,932	)	$	(11,973	)	$	(29,595	)
Foreign	2,240			557			(293		)
	$	(53,692	)	$	(11,416	)	$	(29,888	)

The provision (benefit) for income taxes from continuing operations consisted of the following (in thousands):



	Year ended December 31,
	2017			2016			2015
Provision (benefit) for income taxes:
Current:
Federal	$	(200	)	$	12		$	60
State	115			24			150
Foreign	1,960			1,378			982
Total current	1,875			1,414			1,192
Deferred:
Federal	49,570			(301		)	(7,069		)
State	(4,833		)	(1,007		)	4,962
Foreign	(816		)	338			155
Change in valuation allowance	(64,236		)	2,072			2,767
Total deferred	(20,315		)	1,102			815
Total	$	(18,440	)	$	2,516		$	2,007

A reconciliation of the Company's effective tax rate for continuing operations to the statutory federal rate is as follows:



	Year ended December 31,
	2017		2016		2015
U.S. statutory income tax rate	(35.0	)%	(35.0	)%	(35.0	)%
State income taxes, net of federal benefit	(1.2	)	—		—
Foreign income taxes	0.5		7.9		3.6
Foreign tax audit	—		5.2		—
Acquisition costs	6.0		—		—
Foreign deemed dividends	3.8		5.0		1.7
Stock-based compensation	(26.8	)	38.9		14.4
Tax credits	33.3		(11.6	)	(3.3	)
Uncertain tax positions	1.2		—		—
NOL and credit limitations	18.9		—		—
Valuation allowance	(29.0	)	1.9		24.3
Goodwill amortization	1.7		6.7		2.2
Meals and entertainment	0.5		1.4		0.8
Tax reform	(8.8	)	—		—
Other, net	0.6		1.6		(2.0	)
Effective income tax rate	(34.3	)%	22.0	%	6.7	%

The following is a summary of the significant components of deferred income tax assets and liabilities (in thousands):



	December 31,
	2017			2016
Assets:
Net operating loss carryforwards	$	58,624		$	77,425
Research and development tax credits	24,499			24,440
Other tax credits	—			230
Intangible assets	—			9,270
Deferred revenue	5,886			3,176
Accrued expenses	10,786			6,699
Inventory	5,980			5,010
Stock-based compensation	7,452			14,295
Fixed assets	727			2,879
Other temporary differences	1,556			13
	115,510			143,437
Valuation allowance	(89,600		)	(141,895		)
Total deferred tax assets	25,910			1,542
Liabilities:
Purchased intangible assets	(17,092		)	(3,047		)
Unremitted foreign income	(3,171		)	—
Total deferred tax liabilities	(20,263		)	(3,047		)
Total net deferred tax assets	$	5,647		$	(1,505	)

The deferred tax assets and liabilities based on tax jurisdictions are presented in the Company's consolidated balance sheets as follows:
Deferred income taxes - noncurrent assets	$	8,434		$	1,542
Deferred income taxes - noncurrent liabilities	(2,787		)	(3,047		)
	$	5,647		$	(1,505	)

At December 31, 2017, the Company had cumulative federal and state net operating losses ("NOLs") of $221.5 million. The federal NOL carryforwards expire at various dates from 2020 through 2037. The state NOL carryforwards expire at various dates from 2018 through 2037.

The Company adopted ASU 2016-09 in 2017, which amended the tax accounting for stock-based compensation. Of the federal NOL at December 31, 2016, $150.8 million was attributable to stock option deductions. As of December 31, 2017, the financial reporting NOL and tax NOL were in alignment. At December 31, 2016, the Company's federal NOL carryforwards for tax return purposes were $14.0 million greater than its recognized federal NOL for financial reporting purposes, primarily due to excess tax benefits (stock-based compensation deductions in excess of financial reporting compensation costs) not recognized for financial statement purposes until realized. Prior to the adoption of ASU 2016-09, the tax benefit of this loss would be recognized for financial statement purposes in the period in which the tax benefit reduces income taxes payable, which will not be recognized until the Company recognizes a reduction in taxes payable from all other NOL carryforwards. In addition, the Company had $0.7 million of deferred taxes as of December 31, 2017 related to stock-based compensation expense recognized for financial reporting purposes that is not deductible for tax purposes until options are exercised or shares vest. The ultimate realization of the benefit related to stock options is directly associated with the price of the Company's common stock. Employees will not exercise the underlying options unless the current market price exceeds the option exercise price.

The Company also has available federal and state research and development credit carryforwards of $24.5 million that expire at various dates from 2018 through 2037.

Under the provisions of the Internal Revenue Code, the net operating losses and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating losses and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership of significant shareholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, as well as a similar state provision. The Company incurred a change in ownership as defined in the Internal Revenue Code, and as a result, expects that approximately $31 million of federal and state net operating loss carryforwards and approximately $21 million of research and development tax credits will expire unused. There was no impact to the financial statements due to the Company's full valuation allowance against these assets.

During 2017, the valuation allowance against the deferred tax asset related to net operating losses in Canada was reversed, amounting to a tax benefit approximating $2 million. It had become apparent that the entity had been sufficiently profitable in each of the last three years plus the current year to recognize the deferred tax asset related to the net operating loss carryovers. The valuation allowance was reversed due to the objective verifiable evidence provided by the most recent history of positive income generated by Sonus Canada. The loss carryovers will also be utilized due to the impact of the amalgamation of Sonus Canada into GENBAND Canada. While the Company is recognizing the deferred tax asset related to the net operating losses carried over, it is still carrying a valuation allowance, at 100%, against the SRED credit carryovers associated with the development activity performed in Canada. The newly combined entity has determined that it has excess credit carryover and the objective verifiable evidence is negative in that these credits will likely expire unused.

On December 22, 2017, the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (the "Tax Act"). The Tax Act makes broad and complex changes to the U.S. tax code, including, but not limited to: reducing the U.S. federal corporate tax rate from 35% to 21%; requiring companies to pay a one-time transition tax on certain unrepatriated earnings of foreign subsidiaries; generally eliminating U.S. federal income taxes on dividends from foreign subsidiaries; requiring a current inclusion in U.S. federal taxable income of certain earnings (the Global Intangible Low-taxed Income ("GILTI")) of controlled foreign corporations; eliminating the corporate alternative minimum tax ("AMT") and changing how existing AMT credits can be realized; creating the base erosion anti-abuse tax ("BEAT"); creating a new limitation on deductible interest expense; (8) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017; providing a tax deduction for foreign derived intangible income ("FDII") ; and changing rules related to deductibility of compensation for certain officers.

The impact of certain effects of the Tax Act has been recognized in the period in which the new legislation was enacted per guidance in Staff Accounting Bulletin 118, which allows for a measurement period to complete the accounting for certain elements of the tax reform. The Company has not yet completed a full evaluation of the impact of the Tax Act on its financial statements; however, it has provided a reasonable estimate for the impact related to remeasured deferred tax assets based on the new federal income tax rate of 21%. The Company has also provided for the provisional impact related to the Tax Act's change to the federal NOL and AMT carryovers. The total estimated impact of $4.8 million is reflected in income from continuing operations, and increased the tax benefit for the year ended December 31, 2017. The Company will continue to make and refine its calculations as additional analysis is completed. The Company cannot predict with certainty how the Tax Act will affect the Company's financial position or results of operations.

The Tax Act reduces the corporate tax rate to 21% effective January 1, 2018. Consequently, the Company has recorded a decrease related to net deferred tax assets of $36.5 million and a decrease to the valuation allowance of $38.4 million, with a corresponding provisional net income tax benefit of $1.9 million for the year ended December 31, 2017.

The Tax Act changed the NOL carryover rules and created a new limitation on their use. NOLs created after December 31, 2017 may be carried forwarded indefinitely but are limited to 80% of taxable income in any year. As a result of this change, the Company believes it is appropriate to offset some of its indefinite-lived deferred tax liabilities against its deferred tax assets, and as a result, recognized a $2.8 million estimated benefit in the year ended December 31, 2017.

The Tax Act eliminated the corporate AMT. As a result, the Tax Act made any AMT credit carryovers refundable to the extent not used against the regular tax liability in the 2018 through 2021 tax years. The Company recognized an estimated benefit of $0.1 million in the year ended December 31, 2017 as a result of this change.

The Deemed Repatriation Transition Tax ("Transition Tax") is a tax on previously untaxed accumulated and current earnings and profits ("E&P") of certain of the Company's foreign subsidiaries. To determine the amount of the Transition Tax, the Company must determine, in addition to other factors, the amount of post-1986 E&P of the relevant subsidiaries, as well as the amount of non-U.S. income taxes paid on such earnings. Due to the net E&P deficit of the Company's foreign subsidiaries, the Company does not believe it will incur the Transition Tax and has not recorded a provision Transition Tax obligation.

With regards to the BEAT, the GILTI, the deduction for FDII and other provisions of the Tax Act, the Company will require additional detailed analysis in order to assess its impact.

During 2017 and 2016, the Company performed an analysis to determine if, based on all available evidence, it considered it more likely than not that some portion or all of the recorded deferred tax assets will not be realized in a future period. As a result of the Company's evaluation, the Company concluded that there was insufficient positive evidence to overcome the more objective negative evidence related to its cumulative losses and other factors. Accordingly, the Company has maintained a valuation allowance against its domestic deferred tax asset amounting to $73.1 million at December 31, 2017 and $141.9 million at December 31, 2016. A similar analysis and conclusion was made with regard to the valuation allowance on the deferred tax assets of the Company's Ireland subsidiary, acquired as part of the acquisition of GENBAND, resulting in a valuation allowance of $6.2 million at December 31, 2017. In analyzing the deferred tax assets related to the Company's Canada subsidiaries, the Company concluded that it was more likely than not that the Canadian federal Scientific Research and Experimental Development ("SRED") credits would not be realized in a future period. This resulted in a valuation allowance of $10.3 million.

A reconciliation of the Company's unrecognized tax benefits is as follows (in thousands):



	2017			2016			2015
Unrecognized tax benefits at January 1	$	8,969		$	8,888		$	8,875
Increases related to current year tax positions	139			36			13
Increases related to prior period tax positions	430			723			—
Increases related to business acquisitions	2,012			—			—
Decreases related to prior period tax positions	(7,022		)	(81		)	—
Settlements	—			(597		)	—
Unrecognized tax benefits at December 31	$	4,528		$	8,969		$	8,888

The Company recorded liabilities for potential penalties and interest of $0.2 million for the year ended December 31, 2017, $0.1 million for the year ended December 31, 2016 and $13,000 for the year ended December 31, 2015. The Company had cumulative deferred tax liabilities recorded related to interest and penalties of $0.6 million for the year ended December 31, 2017, $0.2 million for the year ended December 31, 2016 and $0.2 million for the year ended December 31, 2015. Some of the unrecognized tax benefit items are expected to reverse in 2018 due to statute of limitation lapses.

The Company and its subsidiaries file income tax returns in the U.S. federal jurisdiction, as well as various state and foreign jurisdictions. Generally, the tax years 2014 through 2017 remain open to examination by the major taxing jurisdictions to which the Company is subject. The Company's federal NOLs generated prior to 2014 could be adjusted on examination even though the year in which the loss was generated is otherwise closed by the statute of limitations.

As of December 31, 2017, the Company had ongoing income tax audits in certain foreign countries. Management believes that an adequate provision has been recorded for any adjustments that may result from tax examinations.

The 2017 acquisition of GENBAND was accounted for as a non-taxable business combination. GENBAND had previously been treated as a partnership for U.S. tax purposes. Consequently, U.S. federal and state deferred taxes were recorded as part of the business combination based on the differences between the tax basis of the acquired assets and assumed liabilities and their reported amounts for financial reporting purposes. The Company concluded that there was insufficient positive evidence to overcome the more objective negative evidence related to cumulated losses and other factors. The Company recorded a valuation allowance against the acquired deferred tax assets. The Company recorded identifiable intangible assets as part of the purchase accounting for the acquisition. For U.S. tax purposes, the future amortization of these intangibles will be non-deductible, thereby creating income. Since the Company will be filing a consolidated U.S. tax return, the benefit from these identifiable intangible assets will be utilizable. The Company is required to determine its ability to use the tax benefit against the valuation allowance previously established. The Company has determined that it is more likely than not that these benefits will be recognized. As a result, the valuation allowance has been reduced for the assumed net deferred tax liabilities, resulting in an income tax benefit of $16.4 million. This benefit is included as a component of the Company's provision for income taxes for the year ended December 31, 2017.

The 2016 acquisition of Taqua was a taxable purchase of a business under Section 197 of the Internal Revenue Code. The tax amortization related to Taqua goodwill created a deferred tax liability. The 2015 acquisition of the SDN Business was a taxable purchase of a business under Section 197 of the Internal Revenue Code. The tax amortization related to the SDN Business goodwill created a deferred tax liability.

MAJOR CUSTOMERS

12 Months Ended

Dec. 31, 2017

Risks and Uncertainties [Abstract]

MAJOR CUSTOMERS

The following customers contributed 10% or more of the Company's revenue in at least one of the years ended December 31, 2017, 2016 and 2015:



	Year ended December 31,
	2017	2016	2015
Verizon Communications Inc.	17%	*	*
AT&T Inc.	*	12%	13%

At December 31, 2017, two customers accounted for 10% or more of the Company's accounts receivable balance, representing approximately 31% in the aggregate of total accounts receivable. At December 31, 2016, no customer accounted for 10% or more of the Company's accounts receivable balance. The Company performs ongoing credit evaluations of its customers and generally does not require collateral on accounts receivable. The Company maintains an allowance for doubtful accounts and such losses have been within management's expectations.

GEOGRAPHIC AND OPERATING SEGMENT INFORMATION

12 Months Ended

Dec. 31, 2017

Segment Reporting [Abstract]

GEOGRAPHIC INFORMATION

GEOGRAPHIC AND SEGMENT INFORMATION

The Company's classification of revenue by geographic area is determined by the location of the Company's customers. The following table summarizes revenue by geographic area as a percentage of total revenue:



	Year ended December 31,
	2017		2016		2015
United States	66	%	69	%	71	%
Europe, Middle East and Africa	14		13		13
Japan	7		10		10
Other Asia Pacific	7		5		4
Other	6		3		2
	100	%	100	%	100	%

The Company's service revenue is comprised of the following (in thousands):



	Year ended December 31,
	2017		2016		2015
Maintenance	$	114,735	$	86,995	$	89,280
Professional services	34,088		19,215		17,841
	$	148,823	$	106,210	$	107,121

RELATED PARTIES	12 Months Ended
RELATED PARTIES	Dec. 31, 2017
Related Party Transactions [Abstract]
RELATED PARTIES	RELATED PARTIES As a portion of the consideration for the Merger, on October 27, 2017, the Company issued a promissory note for $22.5 million to certain of GENBAND's equity holders who following the Merger, owned greater than five percent of the Company's outstanding shares. As described in Note 12 above, the promissory note does not amortize and the principal thereon is payable in full on the third anniversary of its execution. Interest on the promissory note is payable quarterly in arrears and accrues at a rate of 7.5% per year for the first six months after issuance, and thereafter at a rate of 10% per year. The failure to make any payment under the promissory note when due and, with respect to payment of any interest, the continuation of such failure for a period of thirty days thereafter, constitutes an event of default under the promissory note. If an event of default occurs under the promissory note, the payees may declare the entire balance of the promissory note due and payable (including principal and accrued and unpaid interest) within five business days of the payees' notification to the Company of such acceleration. H. Brian Thompson, who was an independent member of the Company's Board of Directors until the Company's 2016 Annual Meeting on June 9, 2016, is the Executive Chairman of GTT Communications, Inc., a leading global cloud networking provider to multinational clients ("GTT"). Howard Janzen was an independent member of the Company's Board of Directors through the Merger Date and also served as an independent director of GTT. In October 2015, GTT completed the acquisition of One Source Networks Inc., a provider of global data, Internet, SIP trunking and managed services ("One Source"). One Source is a customer of the Company. The Company had a well-established and ongoing business relationship with One Source prior to its acquisition by GTT. The Company recognized revenue aggregating approximately $23,000 from One Source in the period from January 1, 2016 through June 9, 2016, and approximately $150,000 in the year ended December 31, 2015, pursuant to the terms of a contract between the parties, effective June 28, 2010. The Company believes the terms of this contract are consistent with third-party arrangements that provide similar services.

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

Leases

The Company leases its facilities under operating leases, which expire at various times through 2028. The Company is responsible for certain real estate taxes, utilities and maintenance costs under these leases. The Company's corporate headquarters is located in a leased facility in Westford, Massachusetts, consisting of 97,500 square feet under a lease that expires in August 2028.

Escalation clauses, free rent and other lease concessions are recognized on a straight-line basis over the minimum lease term. Rent expense was $5.9 million for the year ended December 31, 2017, $4.5 million for the year ended December 31, 2016 and $5.2 million for the year ended December 31, 2015.

Future minimum payments under operating lease arrangements as of December 31, 2017 were as follows (in thousands):



Years ending December 31,
2018	$	14,703
2019	11,067
2020	6,632
2021	5,612
2022	3,680
Thereafter	15,866
	$	57,560

Litigation and Contingencies

The Company is fully cooperating with an SEC inquiry regarding the development and issuance of Sonus’ first quarter 2015 revenue and earnings guidance. Following recent communications with the SEC's Division of Enforcement (the "Staff"), the Company has reached an agreement in principle to resolve this matter. The Company is negotiating the terms of an order with the Staff in which it will neither admit nor deny, and that as part of the settlement the Company will pay a $1.9 million civil penalty and agree not to violate the securities laws in the future. The Company recorded $1.9 million in the year ended December 31, 2017, including $0.3 million in the three months ended December 31, 2017, for potential fines in connection with this investigation.

In addition, through GENBAND, the Company is involved in three ongoing patent infringement lawsuits with Metaswitch Networks Ltd., Metaswitch Networks Corp. and Metaswitch Inc. (collectively, “Metaswitch”). First, on January 21, 2014, GENBAND and the Company’s indirectly-owned subsidiary, GENBAND US LLC, filed a complaint alleging that Metaswitch infringed certain patents owned by GENBAND. Following unsuccessful mediation, a trial took place and on January 15, 2016 the jury awarded approximately $8,168,000 in past royalty damages to GENBAND, which neither GENBAND nor the Company has recorded. On September 29, 2016, the court confirmed the jury verdict following motions from both parties. GENBAND is seeking additional royalty and other damages and has appealed a ruling of the court denying a permanent injunction against continued infringement by Metaswitch. GENBAND appealed the denial of its request for a permanent injunction and presented oral argument on this issue before the U.S. Court of Appeals for the Federal Circuit on June 8, 2017. On July 10, 2017, the U.S. Court of Appeals for the Federal Circuit vacated the lower court’s denial of a permanent injunction and remanded the case for further consideration of GENBAND’s request for a permanent injunction. On December 18, 2017, the lower court held a hearing on all remaining issues regarding the appeal. All briefing for reconsideration has been completed and the court’s decision is pending.

A second lawsuit was filed on July 7, 2014 by Metaswitch against GENBAND in which both parties asserted claims of patent infringement against each other. The trial for the second lawsuit concluded on March 17, 2016 with no damages awarded to either party. The court entered a final judgment on April 14, 2016 awarding GENBAND its costs as the prevailing party. On August 28, 2017, the court denied all parties’ motions for judgment as a matter of law and for a new trial. On October 26, 2017, the court granted GENBAND’s motion for bill of costs and awarded GENBAND approximately $315,000 against Metaswitch, which was subsequently recorded by the Company in early 2018, when received.

On June 2, 2016, the federal district court signed an order lifting a stay and severing and consolidating various non-patent claims and counterclaims that were previously stayed in the first two lawsuits into a third lawsuit. On March 28, 2017, the court dismissed all parties’ claims in this third lawsuit without prejudice, concluding that it lacked subject-matter jurisdiction over GENBAND’s claims, declining to exercise jurisdiction over Metaswitch’s federal Lanham Act counterclaim, and dismissing Metaswitch’s remaining counterclaims concluding that it lacked subject-matter jurisdiction over those claims. Immediately following the dismissal of this federal court action on March 28, 2017, GENBAND filed a petition against Metaswitch in Texas state court asserting the claims that GENBAND had previously asserted in the federal court action. Metaswitch filed its answer on April 21, 2017 and asserted counterclaims against GENBAND. On August 16, 2017, Metaswitch then amended its counterclaims against GENBAND. A hearing on discovery motions was held on January 4, 2018. The Texas state court has issued a scheduling order setting trial for this case in November 2018. At this time, it is not possible to predict the outcome of the litigation, but the Company does not expect the results of these Metaswitch-related matters to have a material adverse effect on its business or consolidated financial statements.

In addition, the Company is often a party to disputes and legal proceedings that it considers routine and incidental to its business. Management does not expect the results of any of these actions to have a material effect on the Company's business or consolidated financial statements.

QUARTERLY RESULTS (UNAUDITED)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

QUARTERLY RESULTS (UNAUDITED)

The following tables present the Company's quarterly operating results for the years ended December 31, 2017 and 2016. The information for each of these quarters is unaudited and has been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, all necessary adjustments, consisting only of normal recurring adjustments, have been included to present fairly the unaudited consolidated quarterly results when read in conjunction with the Company's audited consolidated financial statements and related notes.



	First Quarter			Second Quarter			Third Quarter		Fourth Quarter (1)
	(In thousands, except per share data)
Fiscal 2017
Revenue	$	53,368		$	55,733		$	74,629	$	146,212
Cost of revenue	19,620			19,331			20,082		69,413
Gross profit	$	33,748		$	36,402		$	54,547	$	76,799
Income (loss) from operations	$	(10,782	)	$	(12,703	)	$	3,919	$	(35,663	)
Net income (loss)	$	(10,646	)	$	(12,345	)	$	3,453	$	(15,714	)
Earnings (loss) per share (3):
Basic	$	(0.22	)	$	(0.25	)	$	0.07	$	(0.18	)
Diluted	$	(0.22	)	$	(0.25	)	$	0.07	$	(0.18	)
Shares used in computing earnings (loss) per share:
Basic	49,114			49,543			49,753		86,567
Diluted	49,114			49,543			50,131		86,567



	First Quarter			Second Quarter			Third Quarter (2)			Fourth Quarter
	(In thousands, except per share data)
Fiscal 2016
Revenue	$	59,151		$	60,857		$	65,011		$	67,572
Cost of revenue	20,748			20,629			21,425			22,178
Gross profit	$	38,403		$	40,228		$	43,586		$	45,394
Loss from operations	$	(3,881	)	$	(2,708	)	$	(4,316	)	$	(2,704	)
Net loss	$	(4,654	)	$	(2,916	)	$	(3,731	)	$	(2,631	)
Loss per share (3):
Basic	$	(0.09	)	$	(0.06	)	$	(0.08	)	$	(0.05	)
Diluted	$	(0.09	)	$	(0.06	)	$	(0.08	)	$	(0.05	)
Shares used in computing loss per share:
Basic	49,484			49,423			49,402			49,232
Diluted	49,484			49,423			49,402			49,232

__________________________________


(1)	Includes the results of GENBAND for the period subsequent to October 27, 2017.


(2)	Includes the results of Taqua for the period subsequent to September 26, 2016.


(3)	Income (loss) per share is calculated independently for each of the quarters presented; accordingly, the sum of the quarterly earnings (loss) per share amounts may not equal the total calculated for the year.

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements have been prepared in United States dollars, in accordance with accounting principles generally accepted in the United States ("GAAP"). On October 27, 2017 (the "Merger Date"), Sonus Networks, Inc. ("Sonus") consummated an acquisition as specified in an Agreement and Plan of Merger (the “Merger Agreement”) with Solstice Sapphire Investments, Inc. ("NewCo") and certain of its wholly-owned subsidiaries, GENBAND Holdings Company, GENBAND Inc. and GENBAND II, Inc. (collectively, "GENBAND") pursuant to which, following a series of merger transactions (collectively, the "Merger"), Sonus and GENBAND each became a wholly-owned subsidiary of NewCo, with Sonus deemed the acquirer in the transaction for accounting purposes. Subsequently, on November 28, 2017, the Company changed its name to "Ribbon Communications Inc." The consolidated financial statements of the Company represent the consolidated financial statements of Sonus, prior to the Merger Date, and the consolidated financial statements of Ribbon, on and after the Merger Date. The financial results of GENBAND are included in Ribbon's consolidated financial statements beginning on the Merger Date.
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements include the accounts of Ribbon and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated in consolidation.
Use of Estimates and Judgments	Use of Estimates and Judgments The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting periods. Significant estimates and judgments relied upon in preparing these consolidated financial statements include accounting for business combinations, revenue recognition for multiple element arrangements, inventory valuations, assumptions used to determine the fair value of stock-based compensation, intangible assets and goodwill valuations, legal contingencies and recoverability of Ribbon's net deferred tax assets and the related valuation allowances. Ribbon regularly assesses these estimates and records changes in estimates in the period in which they become known. Ribbon bases its estimates on historical experience and various other assumptions that it believes to be reasonable under the circumstances.
Business Combinations	Business Combinations The Company recognizes identifiable assets acquired and liabilities assumed at their acquisition date fair values. Goodwill as of the acquisition date is measured as the excess of consideration transferred over the net of the acquisition date fair values of the assets acquired and the liabilities assumed and represents the expected future economic benefits arising from other assets acquired in the business combination that are not individually identified and separately recognized. While the Company uses its best estimates and assumptions as part of the purchase price allocation process to accurately value assets acquired and liabilities assumed at the acquisition date, its estimates are inherently uncertain and subject to refinement. As a result, during the measurement period, which may be up to one year from the acquisition date, the Company records adjustments to the assets acquired and liabilities assumed with the corresponding offset to goodwill to the extent that it identifies adjustments to the preliminary purchase price allocation. Upon the conclusion of the measurement period or final determination of the values of assets acquired and liabilities assumed, whichever comes first, any subsequent adjustments are recorded to the consolidated statements of operations.
Revenue Recognition	Revenue Recognition The Company recognizes revenue from sales when persuasive evidence of an arrangement exists, delivery has occurred, the sale price is fixed or determinable, and collectability of the related receivable is reasonably assured. In instances where customer acceptance is required, revenue is deferred until the acceptance has been achieved. When fees for products or services are not fixed and determinable, the Company defers the recording of receivables, deferred revenue and revenue until such time as the fees become due or are collected. Revenue from maintenance and support services is recognized ratably over the service period. Maintenance revenue is deferred until the associated product is accepted by the customer and all other revenue recognition criteria have been met. Maintenance and support services include telephone support, return and repair support and unspecified rights to product upgrades and enhancements. Revenue from other professional services is typically recognized as the services are delivered if all other revenue recognition criteria have been met. The Company's products typically have both software and non-software components that function together to deliver the products' essential functionality. In addition, hardware sold generally cannot be used apart from the software. Therefore, the Company considers its principal products to be both software and hardware-related. Many of the Company's sales involve multiple element arrangements that include product, maintenance and various professional services. The Company recognizes revenue in accordance with the provisions of Accounting Standards Codification ("ASC") 605-25, Revenue Recognition - Multiple-Element Arrangements ("ASC 605-25") for transactions that include both hardware and software components. The Company recognizes revenue from stand-alone software sales under the software revenue recognition guidance in ASC 985-605, Software - Revenue Recognition ("ASC 985-605"). The Company limits the amount of revenue recognized for delivered elements to the amount that is not contingent on the future delivery of products or services, future performance obligations, or subject to customer-specific return or refund privileges. For multiple-element arrangements that include both software-only products and non-software products, the Company allocates the total arrangement consideration to the software-only deliverables as a group and to the individual non-software deliverables based on their relative selling prices. If an undelivered element (such as maintenance and support services) relates to both the software-only and non-software deliverables, the Company bifurcates the consideration allocated to the undelivered element (such as maintenance and support services) into a non-software component and the software-only component using the relative selling price method. The consideration allocated to the non-software and software-only deliverables is recognized in accordance with the guidance as discussed in this note. Under ASC 985-605, revenue for any undelivered elements that are considered not essential to the functionality of the product and for which VSOE has been established is deferred and recognized upon delivery utilizing the residual method. If the Company has undelivered product for which VSOE has not been established, it defers all revenue on the entire arrangement until VSOE is established or until such elements are delivered, provided that all other revenue recognition criteria are met. If the Company has undelivered services for which VSOE has not been established, the entire arrangement is recognized as revenue over the longest remaining service period from the point in time that all services have commenced and all products have been delivered, provided that all other revenue recognition criteria are met. For transactions that include multiple elements, arrangement consideration is allocated to each element based on the relative selling prices of all of the elements in the arrangement using the fair value hierarchy as required by ASC 605-25. The Company establishes VSOE based upon the price charged when the same element is sold separately or established by management having the relevant pricing authority. The Company has VSOE for certain of its maintenance and support services and certain professional services. When VSOE exists, it is used to determine the selling price of a deliverable. The Company has not been able to establish VSOE of its products, for certain of its services and for certain maintenance offerings because the Company has not sold such products or services on a stand-alone basis, has not priced its products or services within a narrow range, or has limited sales history. When VSOE is not established, the Company attempts to establish the selling price of each element based on TPE. The Company's solution typically differs from that of its peers as there are no similar or interchangeable competitor products or services. The Company's various product, service and maintenance offerings contain a significant level of unique features and functionality and therefore, comparable pricing of competitors' products and services with similar functionality cannot be obtained. Accordingly, the Company is not able to determine TPE for its products or services. When the Company is unable to establish selling price using VSOE or TPE, the Company uses ESP in its allocation of arrangement consideration for the relevant deliverables. The objective of ESP is to determine the price at which the Company would transact a sale if a product or service was sold on a stand-alone basis. The Company determines ESP for its products and certain services by considering multiple factors including, but not limited to, overall market conditions, including geographic or regional-specific market factors, profit objectives and historical pricing practices for such deliverables. The determination of ESP is a formal process within the Company that includes review and approval by the Company's management. Deferred revenue typically includes customer deposits and amounts associated with partial product shipments and maintenance or service contracts. Deferred revenue expected to be recognized as revenue more than one year subsequent to the balance sheet date is reported as a component of long-term liabilities in the consolidated balance sheets. The Company defers recognition of incremental direct costs, such as cost of goods, third-party installations and commissions, until recognition of the related revenue. Such costs are classified as current assets if the deferred revenue is initially classified as current and noncurrent assets if the related deferred revenue is initially classified as long-term. The Company excludes any taxes assessed by a governmental authority that are directly imposed on a revenue-producing transaction (i.e., sales, use and value added) from its revenue and costs. Reimbursement received for out-of-pocket expenses and shipping costs is recorded as revenue. The Company sells the majority of its products directly to its end customers. For products sold to resellers and distributors, the Company recognizes revenue on a sell-through basis.
Financial Instruments	Financial Instruments The carrying amounts of Ribbon's financial instruments, which include cash equivalents, investments, accounts receivable and accounts payable, approximate their fair values. All investments in marketable securities are classified as available-for-sale and are reported at fair value, with unrealized gains and losses excluded from earnings and reported, net of tax, in Accumulated other comprehensive loss, which is a component of stockholders' equity. Unrealized losses that are determined to be other-than-temporary, based on current and expected market conditions, are recognized in earnings. Declines in fair value determined to be credit-related are charged to earnings. The cost of marketable securities sold is determined by the specific identification method. Financial instruments with remaining maturities or that are due within one year from the balance sheet date are classified as current. Financial instruments with remaining maturities or that are payable more than one year from the balance sheet date are classified as noncurrent.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash equivalents are stated at fair value. Cash equivalents are liquid securities that have remaining maturities of three months or less at the date of purchase.
Restricted Cash	Restricted Cash The Company classifies as restricted cash all cash pledged as collateral to secure long-term obligations and all cash whose use is otherwise limited by contractual provisions. Restricted cash is recorded within other current assets on the consolidated balance sheet.
Foreign Currency Translation	Foreign Currency Translation For foreign subsidiaries where the functional currency is the local currency, assets and liabilities are translated into U.S. dollars at the current exchange rate on the balance sheet date. Revenue and expenses are translated at average rates of exchange prevailing during each period. Translation adjustments for these subsidiaries are included in Accumulated other comprehensive loss. For foreign subsidiaries where the functional currency is the U.S. dollar, monetary assets and liabilities are translated into U.S. dollars at the current exchange rate on the balance sheet date. Nonmonetary assets and liabilities are remeasured into U.S. dollars at historical exchange rates. Revenue and expense items are translated at average rates of exchange prevailing during each period. Realized and unrealized foreign currency gains and losses arising from transactions denominated in currencies other than the subsidiary's functional currency are reflected in earnings with the exception of intercompany transactions considered to be of a long-term investment nature.
Inventory	Inventory Inventory is recorded at the lower of cost or market value using the first-in, first-out convention. The Company reduces the carrying value of inventory for those items that are potentially excess, obsolete or slow-moving based on changes in customer demand, technology developments or other economic factors. Ribbon writes down evaluation equipment at the time of shipment to its customers, as it is probable that the inventory value will not be realized. Deferred product costs represent deferred cost of revenue for product shipments to customers prior to satisfaction of Ribbon's revenue recognition criteria. The Company classifies inventory that is not expected to be consumed within one year from the balance sheet date as noncurrent and includes such inventory as a component of Other assets.
Property and Equipment	Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation. Expenditures for maintenance and repairs are charged to expense as incurred. Depreciation is computed using the straight-line method over the estimated useful lives of the related assets, which range from two to five years. Leasehold improvements are amortized over the lesser of the lease term or five years. When an asset is sold or retired, the cost and related accumulated depreciation or amortization are eliminated, and the resulting gain or loss, if any, is recognized in income (loss) from operations in the consolidated statement of operations. The Company reviews property and equipment for impairment in the same manner as intangible assets discussed below. Software development costs associated with internal use software are incurred in three stages of development: the preliminary project stage, the application development stage and the post-implementation stage. Costs incurred during the preliminary project and post-implementation stages are expensed as incurred. Certain qualifying costs incurred during the application development stage are capitalized as property and equipment. Internal use software is amortized on a straight-line basis over its estimated useful life of three years, beginning when the software is ready for its intended use.
Intangible Assets and Goodwill	Intangible Assets and Goodwill Intangible assets are comprised of certain intangible assets arising from the Merger, as well as previous acquisitions. These intangible assets include a combination of in-process research and development, developed technology, customer relationships, trade names, and internal use software. Intangible assets are reviewed for impairment when events or changes in circumstances indicate that their carrying amounts may not be recoverable based upon the estimated undiscounted cash flows. Recoverability of intangible assets with estimated lives and other long-lived assets is measured by a comparison of the carrying amount of an asset or asset group to future net undiscounted cash flows expected to be generated by the asset or asset group. If these comparisons indicate that an asset is not recoverable, the Company will recognize an impairment loss for the amount by which the carrying value of the asset or asset group exceeds the related estimated fair value. Estimated fair value is based on either discounted future operating cash flows or appraised values, depending on the nature of the asset. The Company amortizes its intangible assets over their respective useful lives, with the exception of in-process research and development, which has an indefinite life until the product is generally available, at which time such asset is typically reclassified to developed technology, and the Company begins to amortize this asset. See Note 9 for additional information regarding the Company's intangible assets. Goodwill is recorded when the consideration for an acquisition exceeds the fair value of net tangible and identifiable intangible assets acquired. Goodwill is not amortized, but instead is tested for impairment at least annually, or more frequently if indicators of potential impairment exist, by comparing the fair value of the Company's reporting unit to its carrying value. The Company's annual testing for impairment of goodwill is completed as of November 30. The Company operates as a single operating segment with one reporting unit and consequently evaluates goodwill for impairment based on an evaluation of the fair value of the Company as a whole. The Company performed its step one assessments for each of the years ended December 31, 2017, 2016 and 2015 and determined each year that its market capitalization was significantly in excess of its carrying value and accordingly, there was no impairment of goodwill.
Share-Based Compensation	Stock-Based Compensation The Company's stock-based compensation cost is measured at the grant date based on the fair value of the award and is recognized as expense over the requisite service period, which generally represents the vesting period, and includes an estimate of the awards that will be forfeited. The Company uses the Black-Scholes valuation model for estimating the fair value on the date of grant of stock options. The fair value of stock option awards is affected by the Company's stock price as well as valuation assumptions, including the volatility of Ribbon's stock price, expected term of the option, risk-free interest rate and expected dividends. In 2015, the Company began to grant performance-based stock units ("PSUs") that include a market condition to certain of its executives. The Company uses a Monte Carlo simulation approach to model future stock price movements based upon the risk-free rate of return, the volatility of each entity and the pair-wise covariance between each entity. These results are then used to calculate the grant date fair values of the PSUs.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred.
Advertising Costs	Advertising Costs Advertising costs are expensed as incurred and included as a component of Sales and marketing expense in the Company's consolidated statements of operations.
Operating Segments	Operating Segments The Company operates in a single segment, as the chief operating decision maker makes decisions and assesses performance at the company level. Operating segments are identified as components of an enterprise about which separate discrete financial information is utilized for evaluation by the chief operating decision maker in making decisions regarding resource allocation and assessing performance. To date, the chief operating decision maker has made such decisions and assessed performance at the company level, as one segment. The Company's chief operating decision maker is its President and Chief Executive Officer.
Loss Contingencies and Reserves	Loss Contingencies and Reserves Ribbon is subject to ongoing business risks arising in the ordinary course of business, including legal claims, that affect the estimation process of the carrying value of assets, the recording of liabilities and the possibility of various loss contingencies. An estimated loss contingency is accrued when it is probable that a liability has been incurred or an asset has been impaired and the amount of loss can be reasonably estimated. Ribbon regularly evaluates current information available to determine whether such amounts should be adjusted and records changes in estimates in the period they become known. An allowance for doubtful accounts is estimated based on the Company's assessment of the collectability of specific customer accounts. Ribbon accrues for royalties for technology that it licenses from vendors based on established royalty rates and usage. In certain cases, Ribbon has been contacted by third parties who claim that Ribbon's products infringe on certain intellectual property of a third party. Ribbon evaluates these claims and accrues amounts when it is probable that the obligation has been incurred and the amounts are reasonably estimable.
Accounting for Income Taxes	Accounting for Income Taxes Deferred tax assets and liabilities are recognized for the expected future consequences of events that have been reflected in the consolidated financial statements. Deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax basis of assets and liabilities and operating loss carryforwards, using tax rates expected to be in effect for the years in which the differences are expected to reverse. The Company records valuation allowances to reduce deferred income tax assets to the amount that is more likely than not to be realized. The Company has provided for income taxes on the undistributed earnings of its non-U.S. subsidiaries as of December 31, 2017, with the exception of the Company's Irish subsidiary, as the Company does not plan to permanently reinvest these amounts outside the United States. The repatriation of the undistributed earnings would result in withholding taxes imposed on the repatriation. Consequently, the Company has recorded a tax liability of $3.2 million, primarily consisting of withholding and distribution taxes, relating to undistributed earnings from these subsidiaries as of December 31, 2017. Had the earnings of the Irish subsidiary been determined to not be permanently reinvested outside the U.S., no additional deferred tax liability would be required due to no withholding taxes or income tax expense being imposed on such repatriation. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, no amount of the benefit attributable to the position is recognized. The tax benefit to be recognized of any tax position that meets the more likely than not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), its new standard on revenue from contracts with customers, along with additional ASUs which, among other things, clarified the implementation of the new revenue guidance and delayed the adoption by one year, to January 1, 2018 (collectively, the "New Revenue Standard" or "ASC 606"). The New Revenue Standard outlines a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. The core principle of the revenue model is that an entity recognizes revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In applying the revenue model to contracts within its scope, an entity identifies the contract(s) with a customer, identifies the performance obligations in the contract, determines the transaction price, allocates the transaction price to the performance obligations in the contract and recognizes revenue when (or as) the entity satisfies a performance obligation. Certain of the New Revenue Standard's provisions also apply to transfers of nonfinancial assets, including in-substance nonfinancial assets that are not an output of an entity’s ordinary activities (i.e., property, plant and equipment; real estate; or intangible assets). Existing accounting guidance applicable to these transfers has been amended or superseded. Effective January 1, 2018, the Company adopted the New Revenue Standard using the modified retrospective option and has identified the necessary changes to its policies, processes, systems and controls. However, due to the recent acquisition of GENBAND, the Company has not yet completed all of its internal control procedures. Under the modified retrospective method, the Company is applying the New Revenue Standard to all contracts not yet completed as of January 1, 2018, recognizing in beginning Accumulated deficit an adjustment for the cumulative effect of the change and providing additional disclosures comparing results to those as if the Company was still following the previous accounting standards. Currently, the Company does not have VSOE for certain elements in software bundled arrangements, which results in revenue being recognized ratably over the longest performance period. The majority of the Company's transition adjustment will be related to these arrangements. The Company currently expects that the adjustment to decrease Accumulated deficit as a result of this change will not exceed $15 million. Additionally, the Company expects to capitalize certain commission costs resulting directly from securing contracts which are currently expensed. The Company expects this to result in the capitalization of approximately $1 million of previously expensed commissions expense. In May 2017, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting ("ASU 2017-09"), which amends the scope of modification accounting for share-based payment arrangements such that an entity would not apply modification accounting if the fair value, vesting conditions and classification of the awards are the same immediately before and after the modification. ASU 2017-09 is effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. The Company does not expect the adoption of ASU 2017-09 will have a material impact on its consolidated financial statements. In March 2017, the FASB issued ASU 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post Retirement Benefit Cost ("ASU 2017-07"). ASU 2017-07 amends the requirements in ASC 715 to require entities to disaggregate the current-service-cost component from the other components of net benefit cost (the "other components") and include it with other current compensation costs for related employees, present the other components elsewhere in the income statement and outside of income from operations if such a subtotal is presented and disclose the income statement lines that contain the other components if they are not presented on appropriately described separate lines. ASU 2017-07 is effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. The Company does not expect the adoption of ASU 2017-07 will have a material impact on our consolidated financial statements. In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ("ASU 2017-04"), which removes the requirement to compare the implied fair value of goodwill with its carrying amount as part of step 2 of the goodwill impairment test by instead comparing the fair value of a reporting unit with its carrying amount and recognizing an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value. ASU 2017-04 also clarifies the treatment of foreign currency translation adjustments to reporting units and the income tax effects on goodwill impairment loss measurement. ASU 2017-04 is effective prospectively for the Company beginning January 15, 2020, with early adoption permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company elected to early-adopt ASU 2017-04; such early adoption did not have a material impact on its consolidated financial results. In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory ("ASU 2016-16"), which removes the prohibition in ASC 740, Income Taxes, against the immediate recognition of the current and deferred income tax effects of intra-entity transfers of assets other than inventory. ASU 2016-16 is effective for us beginning January 1, 2019 for both interim and annual reporting periods. The Company does not believe that the adoption of this standard will have a material impact on our consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments ("ASU 2016-15"), which adds or clarifies guidance on eight cash flow issues, including debt prepayment or debt extinguishment costs, settlement of zero-coupon debt instruments or certain other debt instruments, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, proceeds from the settlement of corporate-owned life insurance policies, distributions received from equity method investees, beneficial interests in securitization transactions and separately identifiable cash flows and application of the predominance principle. ASU 2016-15 is effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. Entities must apply the guidance retrospectively to all periods presented but may apply it prospectively from the earliest date practicable if retrospective application would be impracticable. The Company does not expect the adoption of ASU 2016-15 will have a material impact on its consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"), which adds an impairment model that is based on expected losses rather than incurred losses. Under ASU 2016-13, an entity recognizes as an allowance its estimate of expected credit losses, which the FASB believes will result in more timely recognition of such losses. ASU 2016-13 is effective for the Company beginning January 1, 2020 for both interim and annual reporting periods, with early adoption permitted. The Company does not expect the adoption of ASU 2016-13 will have a material impact on its consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting ("ASU 2016-09"), which simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures and statutory tax withholding requirements, as well as classification in the statement of cash flows. ASU 2016-09 became effective for the Company beginning January 1, 2017 for both interim and annual reporting periods. Under ASU 2016-09, the Company will now recognize unrealized excess tax benefits. Due to the full valuation allowance on the Company's federal and state income taxes, the adoption of ASU 2016-09 did not have a material impact on its accounting for income taxes. Without the valuation allowance, the Company would have recognized an increased deferred tax asset approximating $5 million. The Company has elected to continue to apply forfeiture rates to the expense attribution related to stock options, restricted stock awards and restricted stock units, as the Company believes that such continued application results in more accurate expense attribution over the life of these equity grants. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Section A - Leases: Amendments to the FASB Accounting Standards Codification ("ASU 2016-02"), its new standard on accounting for leases. ASU 2016-02 introduces a lessee model that brings most leases onto the balance sheet. ASU 2016-02 eliminates the current GAAP requirement for an entity to use bright-line tests in determining lease classification. ASU 2016-02 is effective for the Company for both interim and annual periods beginning January 1, 2019. Upon adoption of ASU 2016-02, the Company will recognize lease obligations for the right to use these assets in connection with its existing lease agreements. The Company is currently assessing the potential impact of the adoption of ASU 2016-02 on its consolidated financial statements and accordingly, such amounts to be recognized on the balance sheet have yet to be determined. In July 2015, the FASB issued ASU 2015-11, Simplifying the Measurement of Inventory ("ASU 2015-11"), which simplifies the measurement of inventory by requiring entities to measure most inventory at the lower of cost and net realizable value, replacing the previous requirement to measure most inventory at the lower of cost or market. ASU 2015-11 does not apply to inventories that are measured by using either the last-in, first-out method or the retail inventory method. ASU 2015-11 became effective for the Company for both interim and annual reporting periods beginning January 1, 2017. The adoption of ASU 2015-11 did not have a material impact on its consolidated financial statements.

BUSINESS ACQUISITONS (Tables)

12 Months Ended

Dec. 31, 2017

Business Acquisition [Line Items]

Schedule of Components of Acquisition Related Costs

The components of acquisition-related costs incurred in the years ended December 31, 2017, 2016 and 2015 are as follows (in thousands):



	Year ended December 31,
	2017		2016		2015
Professional and services fees (acquisition-related)	$	11,916	$	1,152	$	131
Management bonuses (acquisition-related)	931		—		—
Integration-related expenses	1,916		—		—
	$	14,763	$	1,152	$	131

GENBAND

Business Acquisition [Line Items]

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed

A summary of the preliminary allocation of the purchase consideration for GENBAND is as follows (in thousands):



Fair value of consideration transferred:
Cash consideration:
Repayment of GENBAND long-term debt and accrued interest, related party	$	47,973
Payment of GENBAND management fees due to majority shareholder	10,302
Less cash acquired	(15,324		)
Net cash consideration	42,951
Fair value of Sonus stock issued	413,982
Promissory note issued to GENBAND equity holders	22,500
Fair value of total consideration	$	479,433

Fair value of assets acquired and liabilities assumed:
Current assets, net of cash acquired	$	99,126
Property and equipment	16,770
Intangible assets:
In-process research and development	5,600
Developed technology	129,000
Customer relationships	101,300
Trade names	900
Goodwill	285,825
Other noncurrent assets	6,732
Revolving credit facility	(17,930		)
Deferred revenue	(32,390		)
Other current liabilities	(80,023		)
Deferred revenue, net of current	(6,804		)
Other long-term liabilities	(28,673		)
	$	479,433

Business Acquisition, Pro Forma Information

Accordingly, these unaudited pro forma results are presented for illustrative purposes and are not intended to represent or be indicative of the actual results of operations of the combined company that would have been achieved had the Merger occurred at the beginning of the periods presented, nor are they intended to represent or be indicative of future results of operations (in thousands, except per share amounts):



	Year ended December 31,
	2017			2016
	(unaudited)
Revenue	$	615,286		$	631,914
Net loss	$	(69,741	)	$	(147,394	)
Loss per share	$	(0.69	)	$	(1.46	)

Taqua, LLC

Business Acquisition [Line Items]

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed

A summary of the final allocation of the purchase consideration for Taqua is as follows (in thousands):



Fair value of consideration transferred:
Cash, net of cash acquired	$	19,919

Fair value of assets acquired and liabilities assumed:
Current assets	$	3,347
Property and equipment	1,478
Intangible assets:
Developed technology	2,100
Customer relationships	9,510
Goodwill	9,581
Other noncurrent assets	23
Current liabilities	(5,435		)
Long-term liabilities	(685		)
	$	19,919

SDN Business [Member]

Business Acquisition [Line Items]

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed

A summary of the final allocation of the purchase consideration for the SDN Business is as follows (in thousands):



Fair value of consideration transferred:
Cash, net of cash acquired	$	11,647
Fair value of assets acquired:
Intangible assets:
In-process research and development	$	9,100
Developed technology	1,500
Goodwill	1,047
	$	11,647

CASH EQUIVALENTS AND INVESTMENTS (Tables)

12 Months Ended

Dec. 31, 2017

Investments, All Other Investments [Abstract]

Schedule of amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments

The amortized cost, gross unrealized gains and losses and fair value of the Company's cash equivalents and investments at December 31, 2017 and 2016 were comprised of the following (in thousands):



	December 31, 2017
	Amortized cost		Unrealized gains		Unrealized losses			Fair value
Cash equivalents	$	1,254	$	—	$	—		$	1,254

Short-term investments
U.S. government agency notes	$	4,091	$	—	$	(19	)	$	4,072
Corporate debt securities	8,048		—		(31		)	8,017
Certificates of deposit	5,135		—		—			5,135
	$	17,274	$	—	$	(50	)	$	17,224
Investments
U.S. government agency notes	$	3,992	$	—	$	(28	)	$	3,964
Corporate debt securities	3,908		—		(24		)	3,884
Certificates of deposit	1,183		—		—			1,183
	$	9,083	$	—	$	(52	)	$	9,031



	December 31, 2016
	Amortized cost		Unrealized gains		Unrealized losses			Fair value
Cash equivalents	$	6,619	$	—	$	—		$	6,619

Short-term investments
Municipal obligations	$	3,264	$	—	$	(3	)	$	3,261
U.S. government agency notes	16,477		3		(3		)	16,477
Corporate debt securities	41,893		4		(45		)	41,852
Certificates of deposit	246		—		—			246
	$	61,880	$	7	$	(51	)	$	61,836
Investments
U.S. government agency notes	$	19,473	$	3	$	(39	)	$	19,437
Corporate debt securities	10,520		—		(44		)	10,476
Certificates of deposit	2,458		—		—			2,458
	$	32,451	$	3	$	(83	)	$	32,371

Schedule of fair value of financial assets



			Fair value measurements at December 31, 2017 using:
	Total carrying value at December 31, 2017		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
Cash equivalents	$	1,254	$	1,254	$	—	$	—

Short-term investments
U.S. government agency notes	$	4,072	$	—	$	4,072	$	—
Corporate debt securities	8,017		—		8,017		—
Certificates of deposit	5,135		—		5,135		—
	$	17,224	$	—	$	17,224	$	—
Investments
U.S. government agency notes	$	3,964	$	—	$	3,964	$	—
Corporate debt securities	3,884		—		3,884		—
Certificates of deposit	1,183		—		1,183		—
	$	9,031	$	—	$	9,031	$	—



			Fair value measurements at December 31, 2016 using:
	Total carrying value at December 31, 2016		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
Cash equivalents	$	6,619	$	6,619	$	—	$	—

Short-term investments
Municipal obligations	$	3,261	$	—	$	3,261	$	—
U.S. government agency notes	16,477		—		16,477		—
Corporate debt securities	41,852		—		41,852		—
Certificates of deposit	246		—		246		—
	$	61,836	$	—	$	61,836	$	—
Investments
U.S. government agency notes	$	19,437	$	—	$	19,437	$	—
Corporate debt securities	10,476		—		10,476		—
Certificates of deposit	2,458		—		2,458		—
	$	32,371	$	—	$	32,371	$	—

ACCOUNTS RECEIVABLE, NET (Tables)

12 Months Ended

Dec. 31, 2017

Receivables [Abstract]

Schedule of accounts receivable, net

Accounts receivable, net, consisted of the following (in thousands):



	December 31,
	2017			2016
Accounts receivable	$	165,229		$	53,872
Allowance for doubtful accounts	(73		)	(10		)
Accounts receivable, net	$	165,156		$	53,862

Schedule of allowance for doubtful accounts

The Company's allowance for doubtful accounts activity was as follows (in thousands):



Year ended December 31,	Balance at beginning of year		Charges to expense		Charges (credits) to other accounts (deferred revenue)			Write-offs			Balance at end of year
2017	$	10	$	154	$	(56	)	$	(35	)	$	73
2016	$	10	$	10	$	—		$	(10	)	$	10
2015	$	58	$	17	$	—		$	(65	)	$	10

INVENTORY (Tables)

12 Months Ended

Dec. 31, 2017

Inventory Disclosure [Abstract]

Schedule of inventory

Inventory consisted of the following (in thousands):



	December 31,
	2017			2016
On-hand final assemblies and finished goods inventories	$	18,374		$	15,346
Deferred cost of goods sold	4,569			4,237
	22,943			19,583
Less current portion	(21,303		)	(18,283		)
Noncurrent portion (included in Other assets)	$	1,640		$	1,300

PROPERTY AND EQUIPMENT (Tables)

12 Months Ended

Dec. 31, 2017

Property, Plant and Equipment [Abstract]

Schedule of property and equipment

Property and equipment consisted of the following (in thousands):



		December 31,
	Useful Life	2017			2016
Equipment	2-5 years	$	67,415		$	63,622
Software	2-5 years	21,977			19,378
Furniture and fixtures	3-5 years	1,892			698
Leasehold improvements	Shorter of the life of the lease or estimated useful life (1-5 years)	18,428			11,757
		109,712			95,455
Less accumulated depreciation and amortization		(84,932		)	(83,714		)
Property and equipment, net		$	24,780		$	11,741

Property and equipment under capital leases included in the amounts above were as follows (in thousands):



	December 31,
	2017			2016
Cost	$	664		$	173
Less accumulated depreciation	(180		)	(68		)
Property and equipment under capital leases, net	$	484		$	105

The net book values of the Company's property and equipment by geographic area were as follows (in thousands):



	December 31,
	2017		2016
United States	$	17,576	$	7,939
Canada	1,740		246
Asia/Pacific	3,853		2,963
Europe	1,400		593
Other	211		—
	$	24,780	$	11,741

INTANGIBLE ASSETS AND GOODWILL (Tables)

12 Months Ended

Dec. 31, 2017

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of intangible assets

The Company's intangible assets at December 31, 2017 and 2016 consisted of the following (in thousands):



December 31, 2017	Weighted average amortization period (years)	Cost		Accumulated amortization		Net carrying value
In-process research and development	*	$	5,600	$	—	$	5,600
Developed technology	6.90	153,380		24,211		129,169
Customer relationships	9.32	120,840		12,015		108,825
Trade names	3.00	900		80		820
Internal use software	3.00	730		730		—
	7.77	$	281,450	$	37,036	$	244,414



December 31, 2016	Weighted average amortization period (years)	Cost		Accumulated amortization		Net carrying value
Developed technology	6.54	$	34,980	$	16,453	$	18,527
Customer relationships	5.78	19,540		7,870		11,670
Internal use software	3.00	730		730		—
	6.23	$	55,250	$	25,053	$	30,197

* An in-process research and development intangible asset has an indefinite life until the product is generally available, at which time such asset is typically reclassified to developed technology.

Schedule of amortization expense related to intangible assets

Amortization expense for intangible assets for the years ended December 31, 2017, 2016 and 2015 was as follows (in thousands):



	Year ended December 31,						Statement of operations classification
	2017		2016		2015		Statement of operations classification
Developed technology	$	18,358	$	6,038	$	5,222	Cost of revenue - product
Customer relationships	4,145		1,462		1,723		Sales and marketing
Trade names	80		—		—		Sales and marketing
Internal use software	—		—		162		Cost of revenue - product
	$	22,583	$	7,500	$	7,107

Schedule of estimated future amortization expense for intangible assets

Estimated future amortization expense for the Company's intangible assets at December 31, 2017 was as follows (in thousands):



Years ending December 31,
2018	$	46,013
2019	39,109
2020	38,590
2021	32,472
2022	26,534
Thereafter	61,696
	$	244,414

Schedule of goodwill

The changes in the carrying value of the Company's goodwill in the years ended December 31, 2017 and 2016 were as follows (in thousands):



	Year ended December 31,
	2017			2016
Balance at January 1
Goodwill	$	52,499		$	43,416
Accumulated impairment losses	(3,106		)	(3,106		)
	49,393			40,310
Acquisition of GENBAND	285,825			—
Acquisition of Taqua and subsequent purchase accounting adjustments	498			9,083
Balance at December 31	$	335,716		$	49,393

The components of the Company's goodwill balances at December 31, 2017 and 2016 were as follows:



	December 31,
	2017			2016
Goodwill	$	338,822		$	52,499
Accumulated impairment losses	(3,106		)	(3,106		)
	$	335,716		$	49,393

RESTRUCTURING ACCRUAL (Tables)

12 Months Ended

Dec. 31, 2017

Restructuring and Related Activities [Abstract]

Summary of restructuring accrual activity

Summaries of the Taqua Restructuring Initiative accrual activity for the years ended December 31, 2017 and 2016 are as follows (in thousands):



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	384	$	245	$	(49	)	$	(580	)	$	—
Facilities	218		508		—			(361		)	365
	$	602	$	753	$	(49	)	$	(941	)	$	365



Year ended December 31, 2016	Balance at January 1, 2016		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2016
Severance	$	—	$	971	$	—	$	(587	)	$	384
Facilities	—		218		—		—			218
	$	—	$	1,189	$	—	$	(587	)	$	602

A summary of the Merger Restructuring Initiative accrual activity for the year ended December 31, 2017 is as follows (in thousands):



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2017
Severance	$	—	$	8,508	$	—	$	(913	)	$	7,595

Summaries of the 2016 Restructuring Initiative accrual activity for the years ended December 31, 2017 and 2016 are as follows (in thousands):



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	497	$	405	$	(26	)	$	(876	)	$	—
Facilities	—		126		—			(31		)	95
	$	497	$	531	$	(26	)	$	(907	)	$	95



Year ended December 31, 2016	Balance at January 1, 2016		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2016
Severance	$	—	$	1,484	$	—	$	(987	)	$	497

Summaries of the 2015 Restructuring Initiative accrual for the years ended December 31, 2017 and 2016 are as follows (in thousands):



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2017
Severance	$	168	$	—	$	—	$	(168	)	$	—



Year ended December 31, 2016	Balance at January 1, 2016		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2016
Severance	$	749	$	—	$	67	$	(648	)	$	168

A summary of the GENBAND Restructuring Initiative accrual activity for the year ended December 31, 2017 is as follows (in thousands):



Year ended December 31, 2017	Balance at January 1, 2017		Liability assumed in connection with Merger		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	—	$	3,663	$	—	$	(158	)	$	(1,589	)	$	1,916
Facilities	—		431		—		(123		)	(103		)	205
	$	—	$	4,094	$	—	$	(281	)	$	(1,692	)	$	2,121

STOCK-BASED COMPENSATION PLANS (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of activity related to outstanding stock options

The activity related to the Company's outstanding stock options during the year ended December 31, 2017 was as follows:



	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (in thousands)
Outstanding at January 1, 2016	5,610,106		$	15.73
Granted	7,760		$	6.67
Exercised	(105,688	)	$	5.85
Forfeited	(35,907	)	$	13.82
Expired	(5,041,084	)	$	16.02
Outstanding at December 31, 2017	435,187		$	14.71	4.30	$	66
Vested or expected to vest at December 31, 2017	435,187		$	14.71	4.30	$	66
Exercisable at December 31, 2017	435,187		$	14.71	4.30	$	66

Schedule of assumptions used to estimate the fair value of options at the date of grant using the Black-Scholes option pricing model



	Year ended December 31,
	2017	2016	2015
Risk-free interest rate	1.22% - 1.95%	1.00% - 1.61%	1.46%-1.75%
Expected dividends	—	—	—
Weighted average volatility	51.1%	54.8%	54.3%
Expected life (years)	5.0	5.0-10.0	5.0-6.0

Schedule of activity related to unvested restricted stock grants

The activity related to the Company's RSAs for the year ended December 31, 2017 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2017	2,030,028		$	9.69
Granted	1,763,912		$	6.77
Unvested PSUs converted to RSAs in connection with the Merger	95,834		$	9.44
Vested	(2,080,179	)	$	8.93
Forfeited	(113,013	)	$	8.80
Unvested balance at December 31, 2017	1,696,582		$	7.68

The activity related to the Company's RSUs for the year ended December 31, 2017 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2017	110,219		$	11.95
Granted	—		$	—
Vested	(80,374	)	$	7.06
Forfeited	(11,913	)	$	8.66
Unvested balance at December 31, 2017	17,932		$	6.99

Schedule of activity related to performance stock awards

The activity related to the Company's PSUs for the year ended December 31, 2017 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2017	147,085		$	12.11
Granted	165,000		$	8.45
Unvested PSUs converted to RSAs in connection with the Merger	(95,834	)	$	9.44
Vested	(145,357	)	$	9.44
Forfeited	(10,060	)	$	11.87
Unvested balance at December 31, 2017	60,834		$	9.65

Schedule of stock-based compensation expenses which are included in condensed consolidated statement of operations

The consolidated statements of operations included stock-based compensation for the years ended December 31, 2017, 2016 and 2015 as follows (in thousands):



	Year ended December 31,
	2017		2016		2015
Product cost of revenue	$	514	$	359	$	317
Service cost of revenue	1,448		1,314		1,524
Research and development	7,337		5,014		5,439
Sales and marketing	4,885		6,209		5,423
General and administrative	11,473		6,872		8,996
	$	25,657	$	19,768	$	21,699

Schedule of common stock reserved for future issuance

Common stock reserved for future issuance at December 31, 2017 consists of the following:



Amended and Restated Stock Incentive Plan	5,359,243
ESPP	1,431,513
	6,790,756

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS (Tables)

12 Months Ended

Dec. 31, 2017

Retirement Benefits [Abstract]

Changes in Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status of Plan



Changes in projected benefit obligations:
Projected benefit obligation at Merger Date	$	10,515
Service cost	68
Interest cost	25
Participant contributions	5
Benefits paid	(3		)
Net actuarial loss on obligation	562
Currency loss	312
Projected benefit obligation at December 31, 2017	$	11,484

Changes in plan assets:
Fair value of plan assets at Merger Date	$	3,776
Actual return on plan assets	(8		)
Employer contributions	22
Participant contributions	5
Additional charges	(4		)
Benefits paid	(3		)
Currency gain	105
Fair value of plan assets at December 31, 2017	$	3,893

Funded status at December 31, 2017	$	(7,591	)

Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	$	578

Amounts recognized in the consolidated balance sheet consist of:
Accrued compensation and benefits (current pension liability)	$	(67	)
Other long-term liabilities (non-current pension liability)	(7,524		)
Net amount recognized	$	(7,591	)

Schedule of Accumulated Benefit Obligations

Plans with underfunded or non-funded accumulated benefit obligations at December 31, 2017 are as follows (in thousands):



Aggregate projected benefit obligation	$	11,484
Aggregate accumulated benefit obligation	$	7,793
Aggregate fair value of plan assets	$	3,893

Schedule of Net Benefit Costs

Net periodic benefit costs for the period from the Merger Date to December 31, 2017 are as follows (in thousands):



Service cost	$	68
Interest cost	25
Expected return on plan assets	(8		)
Additional charges	4
Net periodic benefit costs	$	89

Schedule of Expected Benefit Payments

Expected benefit payments for the next ten years are as follows:



Years ending December 31,
2018	$	67
2019	68
2020	49
2021	50
2022	50
2023 to 2027	847
	$	1,131

Schedule of Defined Benefit Plan Amounts Recognized in Other Comprehensive Income (Loss)

The change in plan assets and benefit obligations recognized in other comprehensive loss before tax for the period from the Merger Date to December 31, 2017 was as follows (in thousands):



Net loss	$	578

Schedule of Assumptions Used

The principal weighted average assumptions used to determine the benefit obligation at December 31, 2017 are as follows:



Discount rate	1.31	%
Rate of compensation increase	3.38	%

The principal weighted average assumptions used to determine net period benefit cost for the period from the Merger Date to December 31, 2017 are as follows:



Discount rate	1.49	%
Expected long-term return on plan assets	1.23	%
Rate of compensation increase	3.38	%

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Schedule of Income (Loss) Before Taxes

The components of loss from continuing operations before income taxes consisted of the following (in thousands):



	Year ended December 31,
	2017			2016			2015
Income (loss) before income taxes:
United States	$	(55,932	)	$	(11,973	)	$	(29,595	)
Foreign	2,240			557			(293		)
	$	(53,692	)	$	(11,416	)	$	(29,888	)

Schedule of Income Tax Expense (Benefit)

The provision (benefit) for income taxes from continuing operations consisted of the following (in thousands):



	Year ended December 31,
	2017			2016			2015
Provision (benefit) for income taxes:
Current:
Federal	$	(200	)	$	12		$	60
State	115			24			150
Foreign	1,960			1,378			982
Total current	1,875			1,414			1,192
Deferred:
Federal	49,570			(301		)	(7,069		)
State	(4,833		)	(1,007		)	4,962
Foreign	(816		)	338			155
Change in valuation allowance	(64,236		)	2,072			2,767
Total deferred	(20,315		)	1,102			815
Total	$	(18,440	)	$	2,516		$	2,007

Schedule of Effective Income Tax Rate Reconciliation

A reconciliation of the Company's effective tax rate for continuing operations to the statutory federal rate is as follows:



	Year ended December 31,
	2017		2016		2015
U.S. statutory income tax rate	(35.0	)%	(35.0	)%	(35.0	)%
State income taxes, net of federal benefit	(1.2	)	—		—
Foreign income taxes	0.5		7.9		3.6
Foreign tax audit	—		5.2		—
Acquisition costs	6.0		—		—
Foreign deemed dividends	3.8		5.0		1.7
Stock-based compensation	(26.8	)	38.9		14.4
Tax credits	33.3		(11.6	)	(3.3	)
Uncertain tax positions	1.2		—		—
NOL and credit limitations	18.9		—		—
Valuation allowance	(29.0	)	1.9		24.3
Goodwill amortization	1.7		6.7		2.2
Meals and entertainment	0.5		1.4		0.8
Tax reform	(8.8	)	—		—
Other, net	0.6		1.6		(2.0	)
Effective income tax rate	(34.3	)%	22.0	%	6.7	%

Summary of Deferred Tax Assets and Liabilities

The following is a summary of the significant components of deferred income tax assets and liabilities (in thousands):



	December 31,
	2017			2016
Assets:
Net operating loss carryforwards	$	58,624		$	77,425
Research and development tax credits	24,499			24,440
Other tax credits	—			230
Intangible assets	—			9,270
Deferred revenue	5,886			3,176
Accrued expenses	10,786			6,699
Inventory	5,980			5,010
Stock-based compensation	7,452			14,295
Fixed assets	727			2,879
Other temporary differences	1,556			13
	115,510			143,437
Valuation allowance	(89,600		)	(141,895		)
Total deferred tax assets	25,910			1,542
Liabilities:
Purchased intangible assets	(17,092		)	(3,047		)
Unremitted foreign income	(3,171		)	—
Total deferred tax liabilities	(20,263		)	(3,047		)
Total net deferred tax assets	$	5,647		$	(1,505	)

The deferred tax assets and liabilities based on tax jurisdictions are presented in the Company's consolidated balance sheets as follows:
Deferred income taxes - noncurrent assets	$	8,434		$	1,542
Deferred income taxes - noncurrent liabilities	(2,787		)	(3,047		)
	$	5,647		$	(1,505	)

Schedule of Unrecognized Tax Benefits

A reconciliation of the Company's unrecognized tax benefits is as follows (in thousands):



	2017			2016			2015
Unrecognized tax benefits at January 1	$	8,969		$	8,888		$	8,875
Increases related to current year tax positions	139			36			13
Increases related to prior period tax positions	430			723			—
Increases related to business acquisitions	2,012			—			—
Decreases related to prior period tax positions	(7,022		)	(81		)	—
Settlements	—			(597		)	—
Unrecognized tax benefits at December 31	$	4,528		$	8,969		$	8,888

MAJOR CUSTOMERS (Tables)

12 Months Ended

Dec. 31, 2017

Risks and Uncertainties [Abstract]

Schedule of customers contributing 10% or more of the revenue

The following customers contributed 10% or more of the Company's revenue in at least one of the years ended December 31, 2017, 2016 and 2015:



	Year ended December 31,
	2017	2016	2015
Verizon Communications Inc.	17%	*	*
AT&T Inc.	*	12%	13%

GEOGRAPHIC AND OPERATING SEGMENT INFORMATION (Tables)

12 Months Ended

Dec. 31, 2017

Segment Reporting [Abstract]

Summary of revenue by geographic area as a percentage of total revenue



	Year ended December 31,
	2017		2016		2015
United States	66	%	69	%	71	%
Europe, Middle East and Africa	14		13		13
Japan	7		10		10
Other Asia Pacific	7		5		4
Other	6		3		2
	100	%	100	%	100	%

Schedule of revenue by type

The Company's service revenue is comprised of the following (in thousands):



	Year ended December 31,
	2017		2016		2015
Maintenance	$	114,735	$	86,995	$	89,280
Professional services	34,088		19,215		17,841
	$	148,823	$	106,210	$	107,121

QUARTERLY RESULTS (UNAUDITED) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of quarterly financial information

In the opinion of management, all necessary adjustments, consisting only of normal recurring adjustments, have been included to present fairly the unaudited consolidated quarterly results when read in conjunction with the Company's audited consolidated financial statements and related notes.



	First Quarter			Second Quarter			Third Quarter		Fourth Quarter (1)
	(In thousands, except per share data)
Fiscal 2017
Revenue	$	53,368		$	55,733		$	74,629	$	146,212
Cost of revenue	19,620			19,331			20,082		69,413
Gross profit	$	33,748		$	36,402		$	54,547	$	76,799
Income (loss) from operations	$	(10,782	)	$	(12,703	)	$	3,919	$	(35,663	)
Net income (loss)	$	(10,646	)	$	(12,345	)	$	3,453	$	(15,714	)
Earnings (loss) per share (3):
Basic	$	(0.22	)	$	(0.25	)	$	0.07	$	(0.18	)
Diluted	$	(0.22	)	$	(0.25	)	$	0.07	$	(0.18	)
Shares used in computing earnings (loss) per share:
Basic	49,114			49,543			49,753		86,567
Diluted	49,114			49,543			50,131		86,567



	First Quarter			Second Quarter			Third Quarter (2)			Fourth Quarter
	(In thousands, except per share data)
Fiscal 2016
Revenue	$	59,151		$	60,857		$	65,011		$	67,572
Cost of revenue	20,748			20,629			21,425			22,178
Gross profit	$	38,403		$	40,228		$	43,586		$	45,394
Loss from operations	$	(3,881	)	$	(2,708	)	$	(4,316	)	$	(2,704	)
Net loss	$	(4,654	)	$	(2,916	)	$	(3,731	)	$	(2,631	)
Loss per share (3):
Basic	$	(0.09	)	$	(0.06	)	$	(0.08	)	$	(0.05	)
Diluted	$	(0.09	)	$	(0.06	)	$	(0.08	)	$	(0.05	)
Shares used in computing loss per share:
Basic	49,484			49,423			49,402			49,232
Diluted	49,484			49,423			49,402			49,232

__________________________________


(1)	Includes the results of GENBAND for the period subsequent to October 27, 2017.


(2)	Includes the results of Taqua for the period subsequent to September 26, 2016.


(3)	Income (loss) per share is calculated independently for each of the quarters presented; accordingly, the sum of the quarterly earnings (loss) per share amounts may not equal the total calculated for the year.

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Maximum measurement period from the acquisition date within which company records adjustments to the assets acquired and liabilities assumed	1 year
Advertising expense \| $	$ 0.3	$ 0.1	$ 0.9
Number of reportable operating segments \| segment	1
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Software
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Minimum \| Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	1 year
Minimum \| Software
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Maximum \| Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Maximum \| Software
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Foreign Currency Translation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
General and Administrative Expense [Member]
Schedule of Foreign Currency Balance [Line Items]
Transaction gains (losses)	$ 0.7	$ (0.3)	$ (0.4)

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Increase in deferred tax asset		$ 5,647
Accounting Standards Update 2016-09
Increase in deferred tax asset		$ 5,000
Forecast \| Accounting Standards Update 2014-09
Capitalized commission expense	$ 1,000
Retained Earnings \| Forecast \| Accounting Standards Update 2014-09
Decrease to accumulated deficit	$ (15,000)

BUSINESS ACQUISITONS - GENBAND MERGER (Details) - USD ($)		12 Months Ended
	Oct. 27, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Repayments of lines of credit		$ 13,500,000	$ 0	$ 0
Debt average interest rate		4.51%
Former Sonus Networks Inc. Shareholders [Member]
Business Acquisition [Line Items]
Percentage of ownership after transaction		50.00%
Former GENBAND Holdings Company And Two Related Holding Companies Shareholders [Member]
Business Acquisition [Line Items]
Sale of stock, percentage of ownership after transaction	50.00%
GENBAND
Business Acquisition [Line Items]
Shares issued as consideration in connection with acquisition of GENBAND (in shares)	50,900,000.0	50,857,708
Repayments of long-term debt	$ 47,973,000
Payment of GENBAND management fee	10,302,000
Promissory note issued to GENBAND equity holders	$ 22,500,000.0
Debt average interest rate	4.67%
GENBAND \| Line of Credit
Business Acquisition [Line Items]
Debt assumed as part of GENBAND merger	$ 17,900,000
GENBAND \| Letter of Credit
Business Acquisition [Line Items]
Debt assumed as part of GENBAND merger	$ 2,900,000

BUSINESS ACQUISITONS - SCHEDULE OF GENBAND PURCHASE PRICE ALLOCATION (Details) - USD ($)		12 Months Ended
	Oct. 27, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash consideration:
Net cash consideration		$ 42,951,000	$ 20,669,000	$ 10,897,000
Fair value of Sonus stock issued	$ 413,982,000
Intangible assets:
Goodwill		$ 335,716,000	$ 49,393,000	$ 40,310,000
GENBAND
Cash consideration:
Repayment of GENBAND long-term debt and accrued interest, related party	47,973,000
Payment of GENBAND management fees due to majority shareholder	10,302,000
Less cash acquired	(15,324,000)
Net cash consideration	42,951,000
Promissory note issued to GENBAND equity holders	22,500,000.0
Fair value of total consideration	479,433,000
Fair value of assets acquired and liabilities assumed:
Current assets, net of cash acquired	99,126,000
Property and equipment	16,770,000
Intangible assets:
Goodwill	285,825,000
Other noncurrent assets	6,732,000
Revolving credit facility	(17,930,000)
Deferred revenue	(32,390,000)
Other current liabilities	(80,023,000)
Deferred revenue, net of current	(6,804,000)
Other long-term liabilities	(28,673,000)
Assets acquired and liabilities assumed	479,433,000
GENBAND \| In-process research and development
Intangible assets:
Finite-lived intangibles	5,600,000
GENBAND \| Developed Technology Rights [Member]
Intangible assets:
Finite-lived intangibles	129,000,000
GENBAND \| Customer relationships
Intangible assets:
Finite-lived intangibles	101,300,000
GENBAND \| Trade names
Intangible assets:
Finite-lived intangibles	$ 900,000

BUSINESS ACQUISITONS - GENBAND PRO FORMA RESULTS (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Business Combinations [Abstract]
Revenue	$ 615,286	$ 631,914
Net loss	$ (69,741)	$ (147,394)
Loss per share	$ (0.69)	$ (1.46)

BUSINESS ACQUISITONS - TAQUA, LLC ACQUISITION (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
	Sep. 26, 2016	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Cash, net of cash acquired				$ 42,951	$ 20,669	$ 10,897
Goodwill			$ 49,393	$ 335,716	49,393	$ 40,310
Taqua, LLC
Business Acquisition [Line Items]
Cash, net of cash acquired	$ 19,919
Current assets	10,000
Primary adjustments, increase to current liabilities		$ 400
Primary adjustments, increase to noncurrent liabilities		$ 100
Reversal of contingent liability			10,000
Net reduction in goodwill			(2,200)
Goodwill	9,581
Business combination, pro forma revenue					1,900
Earnings (loss) attributable to acquisition					$ (4,700)
Taqua, LLC \| Developed Technology Rights [Member]
Business Acquisition [Line Items]
Increase (decrease) in intangible assets			(12,100)
Taqua, LLC \| Customer relationships
Business Acquisition [Line Items]
Increase (decrease) in intangible assets			$ 5,500
Taqua, LLC \| Maximum
Business Acquisition [Line Items]
Current assets	$ 65,000

BUSINESS ACQUISITONS - SCHEDULE OF TAQUA, LLC PURCHASE PRICE ALLOCATION (Details) - USD ($) $ in Thousands		12 Months Ended
	Sep. 26, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair value of consideration transferred:
Cash, net of cash acquired		$ 42,951	$ 20,669	$ 10,897
Fair value of assets acquired and liabilities assumed:
Goodwill		$ 335,716	$ 49,393	$ 40,310
Taqua, LLC
Fair value of consideration transferred:
Cash, net of cash acquired	$ 19,919
Fair value of assets acquired and liabilities assumed:
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets	3,347
Property and equipment	1,478
Goodwill	9,581
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets	23
Other current liabilities	(5,435)
Other long-term liabilities	(685)
Assets acquired and liabilities assumed	19,919
Taqua, LLC \| Developed Technology Rights [Member]
Fair value of assets acquired and liabilities assumed:
Intangible assets	2,100
Taqua, LLC \| Customer relationships
Fair value of assets acquired and liabilities assumed:
Intangible assets	$ 9,510

BUSINESS ACQUISITONS - BUSINESS ACQUISITONS - (Details) - USD ($) shares in Millions		3 Months Ended		12 Months Ended
	Jan. 02, 2015	Sep. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 25, 2015	Jul. 02, 2015
Fair value of consideration transferred:
Cash, net of cash acquired				$ 42,951,000	$ 20,669,000	$ 10,897,000
Fair value of assets acquired and liabilities assumed:
Goodwill				335,716,000	49,393,000	40,310,000
Acquisition-Related Costs
Professional and services fees (acquisition-related)				11,916,000	1,152,000	131,000
Management bonuses (acquisition-related)				931,000	0	0
Integration-related expenses				1,916,000	0	0
Acquisition-related				$ 14,763,000	$ 1,152,000	$ 131,000
In-process research and development
Acquisition Of Net
Finite-lived intangible assets, period increase (decrease)		$ (7,500,000)
Treq Labs, Inc [Member]
Acquisition Of Net
Payments to acquire business, net of cash acquired, initial payment	$ 10,100,000
Business combination, future consideration payment								$ 750,000
Business combination, contingent consideration arrangements, shares authorized in earn-out agreement	3.5
Business combination, contingent consideration, liability							$ 0
Fair value of consideration transferred:
Cash, net of cash acquired	$ 11,647,000
Fair value of assets acquired and liabilities assumed:
Goodwill	1,047,000
Assets acquired and liabilities assumed	11,647,000
Treq Labs, Inc [Member] \| In-process research and development
Fair value of assets acquired and liabilities assumed:
Intangible assets	9,100,000
Treq Labs, Inc [Member] \| Developed technology
Fair value of assets acquired and liabilities assumed:
Intangible assets	1,500,000
Goodwill	$ 1,000,000
SDN Business [Member] \| In-process research and development
Acquisition Of Net
Increase (decrease) in intangible assets			$ (1,600,000)
Earn-Out Agreement Revenue Level 1 [Member]
Acquisition Of Net
Business combination, contingent consideration arrangements, duration of earn-out agreement	3 years
Earn-Out Agreement Revenue Level 1 [Member] \| Treq Labs, Inc [Member]
Acquisition Of Net
Business combination, contingent consideration arrangements, shares authorized in earn-out agreement	1.3
Business combination, contingent consideration arrangements, earn-out agreement aggregate revenue threshold	$ 60,000,000
Earn-Out Agreement Revenue Level 2 [Member] \| Treq Labs, Inc [Member]
Acquisition Of Net
Business combination, contingent consideration arrangements, shares authorized in earn-out agreement	2.2
Business combination, contingent consideration arrangements, earn-out agreement aggregate revenue threshold	$ 150,000,000

EARNINGS (LOSS) PER SHARE - (Details) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of weighted average shares outstanding from basic to diluted
Weighted average shares outstanding - basic	86,567	49,753	49,543	49,114	49,232	49,402	49,423	49,484	58,822	49,385	49,560
Potential dilutive common shares									0	0	0
Weighted average shares outstanding - diluted	86,567	50,131	49,543	49,114	49,232	49,402	49,423	49,484	58,822	49,385	49,560
Common stock and unvested shares of restricted stock not included because their effect would have been antidilutive (in shares)									2,500	8,000	8,200

CASH EQUIVALENTS AND INVESTMENTS - Schedule of Activity for Short-Term Investments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Cash equivalents, amortized cost	$ 1,254	$ 6,619
Cash equivalents, fair value	1,254	6,619
Marketable securities, amortized cost	17,274	61,880
Marketable securities, unrealized gains	0	7
Marketable securities, unrealized losses	(50)	(51)
Marketable securities, fair value	17,224	61,836
Investments, amortized cost	9,083	32,451
Investments, unrealized gains	0	3
Investments, unrealized losses	(52)	(83)
Investments, fair value	$ 9,031	32,371
Minimum
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Period considered to classify available-for-sale securities as investments	1 year
Maximum
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Period considered to classify available-for-sale securities as investments	2 years
Municipal obligations
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Marketable securities, amortized cost		3,264
Marketable securities, unrealized gains		0
Marketable securities, unrealized losses		(3)
Marketable securities, fair value		3,261
U.S. government agency notes
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Marketable securities, amortized cost	$ 4,091	16,477
Marketable securities, unrealized gains	0	3
Marketable securities, unrealized losses	(19)	(3)
Marketable securities, fair value	4,072	16,477
Investments, amortized cost	3,992	19,473
Investments, unrealized gains	0	3
Investments, unrealized losses	(28)	(39)
Investments, fair value	3,964	19,437
Corporate debt securities
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Marketable securities, amortized cost	8,048	41,893
Marketable securities, unrealized gains	0	4
Marketable securities, unrealized losses	(31)	(45)
Marketable securities, fair value	8,017	41,852
Investments, amortized cost	3,908	10,520
Investments, unrealized gains	0	0
Investments, unrealized losses	(24)	(44)
Investments, fair value	3,884	10,476
Certificates of deposit
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Marketable securities, amortized cost	5,135	246
Marketable securities, unrealized gains	0	0
Marketable securities, unrealized losses	0	0
Marketable securities, fair value	5,135	246
Investments, amortized cost	1,183	2,458
Investments, unrealized gains	0	0
Investments, unrealized losses	0	0
Investments, fair value	$ 1,183	$ 2,458

CASH EQUIVALENTS AND INVESTMENTS - Schedule of Short-Term Investments by Measurement (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value	$ 1,254	$ 6,619
Marketable securities, fair value	17,224	61,836
Investments, fair value	9,031	32,371
Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value	1,254	6,619
Marketable securities, fair value	0	0
Investments, fair value	0	0
Significant other observable inputs (Level 2)
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value	0	0
Marketable securities, fair value	17,224	61,836
Investments, fair value	9,031	32,371
Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value	0	0
Marketable securities, fair value	0	0
Investments, fair value	0	0
Municipal obligations
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		3,261
Municipal obligations \| Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		0
Municipal obligations \| Significant other observable inputs (Level 2)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		3,261
Municipal obligations \| Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		0
U.S. government agency notes
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	4,072	16,477
Investments, fair value	3,964	19,437
U.S. government agency notes \| Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	0	0
Investments, fair value	0	0
U.S. government agency notes \| Significant other observable inputs (Level 2)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	4,072	16,477
Investments, fair value	3,964	19,437
U.S. government agency notes \| Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	0	0
Investments, fair value	0	0
Corporate debt securities
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	8,017	41,852
Investments, fair value	3,884	10,476
Corporate debt securities \| Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	0	0
Investments, fair value	0	0
Corporate debt securities \| Significant other observable inputs (Level 2)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	8,017	41,852
Investments, fair value	3,884	10,476
Corporate debt securities \| Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	0	0
Investments, fair value	0	0
Certificates of deposit
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	5,135	246
Investments, fair value	1,183	2,458
Certificates of deposit \| Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	0	0
Investments, fair value	0	0
Certificates of deposit \| Significant other observable inputs (Level 2)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	5,135	246
Investments, fair value	1,183	2,458
Certificates of deposit \| Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	0	0
Investments, fair value	$ 0	$ 0

ACCOUNTS RECEIVABLE, NET - Schedule of Accounts Receivabale, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Receivables [Abstract]
Accounts receivable	$ 165,229	$ 53,872
Allowance for doubtful accounts	(73)	(10)	$ (10)	$ (58)
Accounts receivable, net	$ 165,156	$ 53,862

ACCOUNTS RECEIVABLE, NET - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of year	$ 10	$ 10	$ 58
Charges to expense	154	10	17
Charges (credits) to other accounts (deferred revenue)	(56)	0	0
Write-offs	(35)	(10)	(65)
Balance at end of year	$ 73	$ 10	$ 10

INVENTORY - (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Inventory Disclosure [Abstract]
On-hand final assemblies and finished goods inventories	$ 18,374	$ 15,346
Deferred cost of goods sold	4,569	4,237
Gross inventory	22,943	19,583
Less current portion	(21,303)	(18,283)
Noncurrent portion (included in Other assets)	$ 1,640	$ 1,300

PROPERTY AND EQUIPMENT - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 109,712	$ 95,455
Less accumulated depreciation and amortization	(84,932)	(83,714)
Property and equipment, net	$ 24,780	11,741
Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 67,415	63,622
Equipment \| Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Equipment \| Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Software
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Property and equipment, gross	$ 21,977	19,378
Software \| Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Software \| Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,892	698
Furniture and fixtures \| Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Furniture and fixtures \| Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Property and equipment, gross	$ 18,428	$ 11,757
Leasehold improvements \| Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	1 year
Leasehold improvements \| Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years

PROPERTY AND EQUIPMENT - Property and Equipment Under Capital Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Cost	$ 664	$ 173
Less accumulated depreciation	(180)	(68)
Property and equipment under capital leases, net	$ 484	$ 105

PROPERTY AND EQUIPMENT - Property and Equipment by Geographic Area (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 24,780	$ 11,741
United States
Property, Plant and Equipment [Line Items]
Property and equipment, net	17,576	7,939
Canada
Property, Plant and Equipment [Line Items]
Property and equipment, net	1,740	246
Asia/Pacific
Property, Plant and Equipment [Line Items]
Property and equipment, net	3,853	2,963
Europe
Property, Plant and Equipment [Line Items]
Property and equipment, net	1,400	593
Other
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 211	$ 0

INTANGIBLE ASSETS AND GOODWILL - (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 27, 2017	Dec. 31, 2016	Sep. 26, 2016
Intangible Assets And Goodwill
Weighted average amortization period (years)		7 years 9 months 7 days	6 years 2 months 24 days
Cost	$ 281,450					$ 55,250
Accumulated amortization	37,036					25,053
Net carrying value	244,414					30,197
Amortization expense		$ 17,112	$ 7,500	$ 7,107
Amortization and impairment of intangible assets		22,583
Estimated future amortization expense for intangible assets
2018	46,013
2019	39,109
2020	38,590
2021	32,472
2022	26,534
Thereafter	61,696
Total	244,414
Impairment of intangible assets		5,471	0	0
Goodwill [Roll Forward]
Goodwill	338,822			43,416		52,499
Accumulated impairment losses	(3,106)			(3,106)		(3,106)
Goodwill at the beginning of the period		49,393	40,310
Goodwill at the end of the period	335,716	$ 49,393	$ 40,310	40,310		49,393
In-process research and development
Intangible Assets And Goodwill
Cost	5,600
Accumulated amortization	0
Net carrying value	5,600
Developed technology
Intangible Assets And Goodwill
Weighted average amortization period (years)		6 years 10 months 25 days	6 years 6 months 15 days
Cost	153,380					34,980
Accumulated amortization	24,211					16,453
Net carrying value	129,169					18,527
Amortization expense		$ 18,358	$ 6,038	5,222
Estimated future amortization expense for intangible assets
Impairment of intangible assets	5,500
Customer relationships
Intangible Assets And Goodwill
Weighted average amortization period (years)		9 years 3 months 26 days	5 years 9 months 10 days
Cost	120,840					19,540
Accumulated amortization	12,015					7,870
Net carrying value	108,825					11,670
Amortization expense		$ 4,145	$ 1,462	1,723
Trade names
Intangible Assets And Goodwill
Weighted average amortization period (years)		3 years
Cost	900
Accumulated amortization	80
Net carrying value	820
Amortization expense		$ 80	$ 0	0
Internal use software
Intangible Assets And Goodwill
Weighted average amortization period (years)		3 years	3 years
Cost	730					730
Accumulated amortization	730					730
Net carrying value	$ 0					$ 0
Amortization expense		$ 0	$ 0	$ 162
GENBAND
Goodwill [Roll Forward]
Acquisition of NET		285,825	0
Goodwill at the end of the period					$ 285,825
Taqua, LLC
Goodwill [Roll Forward]
Acquisition of NET		$ 498	$ 9,083
Goodwill at the end of the period							$ 9,581

ACCRUED EXPENSES - (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Employee compensation and related costs	$ 37,782	$ 15,879
Professional fees	13,743	1,243
Other	24,855	9,918
Total	$ 76,380	$ 27,040

RESTRUCTURING ACCRUAL - (Details) $ in Thousands		8 Months Ended	12 Months Ended
RESTRUCTURING ACCRUAL - (Details) $ in Thousands	Apr. 16, 2015 employees	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			$ 1,267
Restructuring			9,436		$ 2,740	$ 2,148
Balance at the end of the period			10,176		1,267
Accrued restructuring			200		100
2016 Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			497
Restructuring			531	$ 2,000	1,500
Adjustments for changes in estimate			(26)
Cash payments			(907)
Balance at the end of the period			95		497
2016 Restructuring \| Employee severance
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			497		0
Restructuring			405	1,900	1,484
Adjustments for changes in estimate			(26)		0
Cash payments			(876)		(987)
Balance at the end of the period			0		497	0
2016 Restructuring \| Facilities
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			0
Restructuring			126	$ 100
Adjustments for changes in estimate			0
Cash payments			(31)
Balance at the end of the period			95		0
Taqua Restructuring Incentive
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			602		0
Restructuring		$ 1,800	753		1,189
Other Restructuring Costs			700
Adjustments for changes in estimate			(49)		0
Cash payments			(941)		(587)
Balance at the end of the period			365		602	0
Taqua Restructuring Incentive \| Employee severance
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			384		0
Restructuring		1,200	245		971
Adjustments for changes in estimate			(49)		0
Cash payments			(580)		(587)
Balance at the end of the period			0		384	0
Taqua Restructuring Incentive \| Facilities
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			218		0
Restructuring		$ 600	508		218
Adjustments for changes in estimate			0		0
Cash payments			(361)		0
Balance at the end of the period			365		218	0
2015 Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated \| employees	150
Number of positions eliminated (percent)	12.50%
2015 Restructuring [Member] \| Employee severance
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			168		749
Restructuring			0		0	3,800
Adjustments for changes in estimate			0		67
Cash payments			(168)		(648)
Balance at the end of the period			$ 0		$ 168	$ 749

RESTRUCTURING ACCRUAL - Merger Restructuring Initiative (Details) $ in Thousands		12 Months Ended
	Oct. 27, 2017 USD ($)	Dec. 31, 2017 USD ($) employees	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period		$ 1,267
Liability assumed in connection with Merger	$ 4,094
Initiatives charged to expense		9,436	$ 2,740	$ 2,148
Balance at the end of the period		$ 10,176	1,267
Merger Restructuring
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated \| employees		120
Additional positions expected to be eliminated in 2018 \| employees		40
Expected future cost		$ 12,000
GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period		0
Initiatives charged to expense		0
Adjustments for changes in estimate		(281)
Cash payments		(1,692)
Balance at the end of the period		2,121	0
GENBAND
Restructuring Cost and Reserve [Line Items]
Restructuring liability assumed	4,100
Employee severance \| Merger Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period		0
Initiatives charged to expense		8,508
Adjustments for changes in estimate		0
Cash payments		(913)
Balance at the end of the period		7,595	0
Employee severance \| GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period		0
Initiatives charged to expense		0
Adjustments for changes in estimate		(158)
Cash payments		(1,589)
Balance at the end of the period		1,916	0
Employee severance \| GENBAND
Restructuring Cost and Reserve [Line Items]
Restructuring liability assumed	3,700
Restructuring Reserve [Roll Forward]
Liability assumed in connection with Merger	3,663
Facilities \| GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period		0
Initiatives charged to expense		0
Adjustments for changes in estimate		(123)
Cash payments		(103)
Balance at the end of the period		$ 205	$ 0
Facilities \| GENBAND
Restructuring Cost and Reserve [Line Items]
Restructuring liability assumed	400
Restructuring Reserve [Roll Forward]
Liability assumed in connection with Merger	$ 431

DEBT - Assumed Senior Secured Credit Agreement (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 21, 2017	Oct. 27, 2017
Debt Instrument [Line Items]
Debt average interest rate	4.51%
Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Line of credit potential increases		$ 100.0
Silicon Valley Bank \| Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Current borrowing capacity for line of credit	$ 50.0
Potential further increases	75.0
Line of credit potential increases	$ 125.0
GENBAND
Debt Instrument [Line Items]
Debt average interest rate			4.67%
Line of Credit \| GENBAND
Debt Instrument [Line Items]
Debt assumed as part of GENBAND merger			$ 17.9
Letter of Credit \| GENBAND
Debt Instrument [Line Items]
Debt assumed as part of GENBAND merger			$ 2.9

DEBT - Senior Secured Credit Facility (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 21, 2017
Debt Instrument [Line Items]
Outstanding debt	$ 20.0
Debt interest rate	4.51%
Line of Credit
Debt Instrument [Line Items]
Commitment potential aggregate increase amount		$ 50.0
Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Commitments from lender		100.0
Fronting fee percentage	0.125%
Default rate percentage	2.00%
Revolving Credit Facility \| Line of Credit \| Minimum
Debt Instrument [Line Items]
Commitment fee percentage	0.25%
Letter of credit, fee multiplier	1.50%
Revolving Credit Facility \| Line of Credit \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
Commitment fee percentage	0.40%
Letter of credit, fee multiplier	2.00%
Revolving Credit Facility \| Line of Credit \| London Interbank Offered Rate (LIBOR) \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%
Revolving Credit Facility \| Line of Credit \| London Interbank Offered Rate (LIBOR) \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.00%
Revolving Credit Facility \| Line of Credit \| Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Revolving Credit Facility \| Line of Credit \| Additional Applicable Margin \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Letter of Credit \| Line of Credit
Debt Instrument [Line Items]
Commitments from lender		15.0
Outstanding debt	$ 2.9
Debt interest rate	2.00%
Swingline loan \| Line of Credit
Debt Instrument [Line Items]
Commitments from lender		$ 15.0

DEBT - Promissory Note (Details) - GENBAND - USD ($)	Dec. 31, 2017	Oct. 27, 2017
Debt Instrument [Line Items]
Promissory note issued to GENBAND equity holders		$ 22,500,000.0
Minimum
Debt Instrument [Line Items]
Promissory note interest rate	7.50%
Maximum
Debt Instrument [Line Items]
Promissory note interest rate	10.00%

LONG-TERM LIABILITIES - Schedule of Long-Term Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Liabilities [Abstract]
Capital lease obligations	$ 837	$ 124
Deferred rent	1,359	1,812
Restructuring	10,176	1,267
Pension obligations	7,524	0
Taxes payable	2,079	0
Other	2,544	790
Long-term liabilities	24,519	3,993
Current portion	(11,330)	(2,360)
Long-term liabilities, net of current portion	$ 13,189	$ 1,633

COMMON STOCK REPURCHASES (Details) - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 29, 2013
Class of Stock [Line Items]
Payments for repurchase of common stock	$ 0	$ 9,530,000	$ 7,917,000
Common Stock
Class of Stock [Line Items]
Authorized amount				$ 100,000,000
Payments for repurchase of common stock		$ 9,500,000
Shares repurchased and retired, shares		1.3

STOCK-BASED COMPENSATION PLANS - Amended and Restate Stock Incentive Plan (Details) - shares	Jun. 09, 2016	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)		6,790,756
Amended and Restated Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional authorized shares	900,000

STOCK-BASED COMPENSATION PLANS - Treatment of Equity Awards in Connection with the Merger (Details) - shares shares in Thousands				12 Months Ended
	Oct. 27, 2017	Oct. 25, 2017	Oct. 22, 2017	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated vesting (in shares)			300
Canceled vested unexervised stock options (in shares)		4,500
RSA
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated vesting (in shares)	1,100
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated vesting (in shares)	36
PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated vesting (in shares)				98

STOCK-BASED COMPENSATION PLANS - Executive Equity Arrangements (Details) $ in Millions						1 Months Ended	9 Months Ended	12 Months Ended
	Apr. 01, 2017 shares	Mar. 31, 2017 shares	Mar. 16, 2017 shares	Apr. 01, 2016 executives shares	Mar. 16, 2015 executives shares	Feb. 28, 2017 shares	Sep. 25, 2015 USD ($) executives	Dec. 31, 2017 shares
PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)				131,250	131,250			165,000
Executive granted shares \| executives				6	8
Vested (in shares)								(145,357)
Forfeited (in shares)								10,060
2017 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)		165,000
2016 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of performance metrics achieved						90.40%
Vested (in shares)						(24,106)
Forfeited (in shares)			2,560
2015 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of performance metrics achieved						76.00%
Vested (in shares)						(23,750)
Forfeited (in shares)	7,500
2015 Bonus
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of executives \| executives							22
Stock-based compensation expense, adjustments to additional paid in capital \| $							$ 1.0
Deferred Bonus \| 2015 Bonus
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Bonus expense \| $							$ 1.3

STOCK-BASED COMPENSATION PLANS - Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from the exercise of stock options	$ 617	$ 153	$ 1,757
Amended 2007 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	10 years
2008 plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	7 years
2012 plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	5 years
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value	$ 3.05	$ 4.39	$ 7.30
Intrinsic value of options exercised	$ 200	$ 42	$ 900
Proceeds from the exercise of stock options	$ 600	$ 200	$ 1,800

STOCK-BASED COMPENSATION PLANS - Other Disclosures (Details) - USD ($)				3 Months Ended	12 Months Ended
	Dec. 13, 2017	Apr. 01, 2016	Mar. 16, 2015	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted average exercise price
Granted (in dollars per share)					$ 6.67
Range of assumptions used in estimating fair value of options
Expected dividends					0.00%	0.00%	0.00%
Expected life					5 years
Weighted average grant-date fair value
Total fair value of restricted stock awards, restricted stock units, performance-based stock awards and performance-based stock units on date vested					$ 20,515,000	$ 10,376,000	$ 9,138,000
Shares available for future issuance (in shares)					6,790,756
Tax benefit for employee stock-based compensation					$ 0	0	0
Stock-based compensation (in dollars)					25,657,000	$ 19,768,000	$ 21,699,000
Unrecognized stock-based compensation expense					$ 7,500,000
Weighted average period (in years)					2 years
Minimum
Range of assumptions used in estimating fair value of options
Expected life						5 years	5 years
Maximum
Range of assumptions used in estimating fair value of options
Expected life						10 years	6 years
Product cost of revenue [Member]
Weighted average grant-date fair value
Stock-based compensation (in dollars)					$ 514,000	$ 359,000	$ 317,000
Service cost of revenue [Member]
Weighted average grant-date fair value
Stock-based compensation (in dollars)					1,448,000	1,314,000	1,524,000
Research and development [Member]
Weighted average grant-date fair value
Stock-based compensation (in dollars)					7,337,000	5,014,000	5,439,000
Sales and marketing [Member]
Weighted average grant-date fair value
Stock-based compensation (in dollars)					4,885,000	6,209,000	5,423,000
General and administrative [Member]
Weighted average grant-date fair value
Stock-based compensation (in dollars)					$ 11,473,000	$ 6,872,000	$ 8,996,000
Stock options
Number of shares
Outstanding at the beginning of the period (in shares)				5,610,106	5,610,106
Granted (in shares)					7,760
Exercised (in shares)					(105,688)
Forfeited (in shares)					(35,907)
Expired (in shares)					(5,041,084)
Outstanding at the end of the period (in shares)					435,187	5,610,106
Vested or expected to vest at the end of the period (in shares)					435,187
Exercisable at the end of the period (in shares)					435,187
Weighted average exercise price
Outstanding at the beginning of the period (in dollars per share)				$ 15.73	$ 15.73
Exercised (in dollars per share)					5.85
Forfeited (in dollars per share)					13.82
Expired (in dollars per share)					16.02
Outstanding at the end of the period (in dollars per share)					14.71	$ 15.73
Vested or expected to vest at the end of the period (in dollars per share)					14.71
Exercisable at the end of the period (in dollars per share)					$ 14.71
Weighted average remaining contractual life (in years)
Outstanding at the end of the period					4 years 3 months 20 days
Vested or expected to vest at the end of the period					4 years 3 months 20 days
Exercisable at the end of the period					4 years 3 months 20 days
Aggregate intrinsic value (in dollars)
Outstanding at the end of the period (in dollars)					$ 66,000
Vested or expected to vest at the end of the period (in dollars)					66,000
Exercisable at the end of the period (in dollars)					$ 66,000
Range of assumptions used in estimating fair value of options
Risk-free interest rates, minimum					1.22%	1.00%	1.46%
Risk-free interest rates, maximum					1.95%	1.61%	1.75%
Weighted average volatility					51.10%	54.80%	54.30%
Weighted average grant-date fair value
Stock-based compensation (in dollars)	$ 1,600,000
Stock options \| Sales and marketing [Member] \| Restatement Adjustment [Member]
Weighted average grant-date fair value
Stock-based compensation (in dollars)				$ (1,000,000)
RSA
Change in unvested restricted stock awards
Unvested balance at the beginning of the period (in shares)				2,030,028	2,030,028
Granted (in shares)					1,763,912
Converted (in period)					95,834
Vested (in shares)					2,080,179
Forfeited (in shares)					(113,013)
Unvested balance at the end of the period (in shares)					1,696,582	2,030,028
Weighted average grant-date fair value
Unvested balance at the end of the period (in dollars per share)				$ 9.69	$ 9.69
Granted (in dollars per share)					6.77
Converted (in dollars per share)					9.44
Vested (in dollars per share)					8.93
Forfeited (in dollars per share)					8.80
Unvested balance at end of the period (in dollars per share)					$ 7.68	$ 9.69
Total fair value of restricted stock awards, restricted stock units, performance-based stock awards and performance-based stock units on date vested					$ 19,100,000	$ 10,100,000	$ 8,500,000
Restricted Stock Awards [Member]
Change in unvested restricted stock awards
Unvested balance at the beginning of the period (in shares)				110,219	110,219
Granted (in shares)					0
Vested (in shares)					80,374
Forfeited (in shares)					(11,913)
Unvested balance at the end of the period (in shares)					17,932	110,219
Weighted average grant-date fair value
Unvested balance at the end of the period (in dollars per share)				$ 11.95	$ 11.95
Granted (in dollars per share)					0.00
Vested (in dollars per share)					7.06
Forfeited (in dollars per share)					8.66
Unvested balance at end of the period (in dollars per share)					$ 6.99	$ 11.95
PSAs
Change in unvested restricted stock awards
Unvested balance at the beginning of the period (in shares)				147,085	147,085
Granted (in shares)		131,250	131,250		165,000
Converted (in period)					(95,834)
Vested (in shares)					145,357
Forfeited (in shares)					(10,060)
Unvested balance at the end of the period (in shares)					60,834	147,085
Weighted average grant-date fair value
Unvested balance at the end of the period (in dollars per share)				$ 12.11	$ 12.11
Granted (in dollars per share)					8.45
Converted (in dollars per share)					9.44
Vested (in dollars per share)					9.44
Forfeited (in dollars per share)					11.87
Unvested balance at end of the period (in dollars per share)					$ 9.65	$ 12.11
Total fair value of restricted stock awards, restricted stock units, performance-based stock awards and performance-based stock units on date vested					$ 1,400,000	$ 200,000	$ 600,000
Amended 2007 Plan
Weighted average grant-date fair value
Shares available for future issuance (in shares)					5,359,243
ESPP [Member]
Weighted average grant-date fair value
Maximum shares per employee					500
Number of shares authorized					5,000,000
Shares available for future issuance (in shares)					1,431,513
GENBAND \| Stock options
Weighted average grant-date fair value
Stock-based compensation (in dollars)					$ 8,600,000

EMPLOYEE DEFINED CONTRIBUTION PLANS - (Details) - USD ($)		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Maximum amount of employer match per employee		$ 2,000
Meximum semi-annual amount of employer match per employee		1,000
Defined contribution expense		$ 1,400,000	$ 600,000
Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Employer matching percent of employees' contribution	50.00%
Employer matching contribution percentage of employees' gross pay	4.00%
Maximum employer match percentage per employee	2.00%

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Changes in Benefit Obligations and Fair Value of Plan (Details) $ in Thousands	2 Months Ended
	Dec. 31, 2017 USD ($)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning balance	$ 10,515
Service cost	68
Interest cost	25
Participant contributions	5
Benefits paid	(3)
Net actuarial loss on obligation	562
Currency loss	312
Ending balance	11,484
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value beginning balance	3,776
Actual return on plan assets	(8)
Employer contributions	22
Participant contributions	5
Additional charges	(4)
Benefits paid	(3)
Currency gain	105
Fair value ending balance	3,893
Funded status at December 31, 2017	(7,591)
Net actuarial loss	578
Liability, Defined Benefit Plan [Abstract]
Accrued compensation and benefits (current pension liability)	(67)
Other long-term liabilities (non-current pension liability)	(7,524)
Net amount recognized	$ (7,591)

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Accumulated Benefit Obligation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Oct. 27, 2017
Retirement Benefits [Abstract]
Aggregate projected benefit obligation	$ 11,484
Aggregate accumulated benefit obligation	7,793
Aggregate fair value of plan assets	$ 3,893	$ 3,776

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Net Periodic Benefit Costs (Details) $ in Thousands	2 Months Ended
	Dec. 31, 2017 USD ($)
Retirement Benefits [Abstract]
Service cost	$ 68
Interest cost	25
Expected return on plan assets	(8)
Additional charges	4
Net periodic benefit costs	$ 89

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS (Details) - USD ($)	2 Months Ended
	Dec. 31, 2017	Oct. 27, 2017
Retirement Benefits [Abstract]
Benefits paid	$ 3,000
Net gains and losses amortization threshold	10.00%
Expected amortization of gain (loss) in the next fiscal year	$ 0
Fair value of combined plan assets	3,893,000	$ 3,776,000
Participant contributions	$ 5,000
Fixed contributions per employee, percent	5.00%
Employer pension plan contributions	$ 22,000
Expected future employer contributions to pension plans in 2018	$ 200,000

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Expected Future Benefit Payments (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Retirement Benefits [Abstract]
2018	$ 67
2019	68
2020	49
2021	50
2022	50
2023 to 2027	847
Expected future benefit payments	$ 1,131

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Change Recognized in Other Comprehesinve Loss (Details) $ in Thousands	2 Months Ended
	Dec. 31, 2017 USD ($)
Retirement Benefits [Abstract]
Net loss	$ 578

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Assumptions for Benefit Obligation (Details)	Dec. 31, 2017
Retirement Benefits [Abstract]
Discount rate	1.31%
Rate of compensation increase	3.38%

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Assumption for Net Periodic Benefit Cost (Details)	2 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
Discount rate	1.49%
Expected long-term return on plan assets	1.23%
Rate of compensation increase	3.38%

INCOME TAXES - Schedule of Income (Loss) Before Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss before income taxes:
United States	$ (55,932)	$ (11,973)	$ (29,595)
Foreign	2,240	557	(293)
Loss before income taxes	$ (53,692)	$ (11,416)	$ (29,888)

INCOME TAXES - Schedule of Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ (200)	$ 12	$ 60
State	115	24	150
Foreign	1,960	1,378	982
Total current	1,875	1,414	1,192
Deferred:
Federal	49,570	(301)	(7,069)
State	(4,833)	(1,007)	4,962
Foreign	(816)	338	155
Change in valuation allowance	(64,236)	2,072	2,767
Total deferred	(20,315)	1,102	815
Total	$ (18,440)	$ 2,516	$ 2,007

INCOME TAXES - Schedule of Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. statutory income tax rate	(35.00%)	(35.00%)	(35.00%)
State income taxes, net of federal benefit	(1.20%)	0.00%	0.00%
Foreign income taxes	0.50%	7.90%	3.60%
Foreign tax audit	0.00%	5.20%	0.00%
Acquisition costs	6.00%	0.00%	0.00%
Foreign deemed dividends	3.80%	5.00%	1.70%
Stock-based compensation	(26.80%)	38.90%	14.40%
Tax credits	33.30%	(11.60%)	(3.30%)
Uncertain tax positions	1.20%	0.00%	0.00%
NOL and credit limitations	18.90%	0.00%	0.00%
Valuation allowance	(29.00%)	1.90%	24.30%
Goodwill amortization	1.70%	6.70%	2.20%
Meals and entertainment	0.50%	1.40%	0.80%
Tax reform	(8.80%)	0.00%	0.00%
Other, net	0.60%	1.60%	(2.00%)
Effective income tax rate	(34.30%)	22.00%	6.70%

INCOME TAXES - Summary of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Net operating loss carryforwards	$ 58,624	$ 77,425
Research and development tax credits	24,499	24,440
Other tax credits	0	230
Intangible assets	0	9,270
Deferred revenue	5,886	3,176
Accrued expenses	10,786	6,699
Inventory	5,980	5,010
Stock-based compensation	7,452	14,295
Fixed assets	727	2,879
Other temporary differences	1,556	13
Deferred tax assets, gross	115,510	143,437
Valuation allowance	(89,600)	(141,895)
Total deferred tax assets	25,910	1,542
Liabilities:
Purchased intangible assets	(17,092)	(3,047)
Unremitted foreign income	(3,171)	0
Total deferred tax liabilities	(20,263)	(3,047)
Total net deferred tax assets	5,647
Total net deferred tax assets		(1,505)
Deferred income taxes - noncurrent assets	8,434	1,542
Deferred income taxes - noncurrent liabilities	$ (2,787)	$ (3,047)

INCOME TAXES - Narrative (Details) - USD ($)	12 Months Ended
INCOME TAXES - Narrative (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Deferred tax assets, stock-based compensation	$ 7,452,000	$ 14,295,000
Tax credit carryforward	$ 24,500,000
Significant change in shareholder ownership, period of change	3 years
Significant change in shareholder ownership, benchmark percentage	50.00%
Net operating loss carryforwards expected to expire unused	$ 31,000,000
Research and development tax credit carryforward expected to expire unused	21,000,000
Income tax benefit	18,440,000	(2,516,000)	$ (2,007,000)
Deferred tax assets, valuation allowance	89,600,000	141,895,000
Tax Act impact on tax benefit	4,800,000
Decrease in net deferred tax asset as a result of Tax Act	(36,500,000)
Decrease in valuation allowance	38,400,000
Income tax benefit, tax rate change as a result of Tax Act	1,900,000
Income tax benefit as a result of Tax Act	2,800,000
Income tax benefit as a result Tax Act change in AMT credit carryover	100,000
Potential penalties and interest	200,000	100,000	13,000
Interest and penalties accrued in deferred tax liability	600,000	200,000	$ 200,000
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	221,500,000
Portion of operating loss carryforwards for stock option deductions		150,800,000
Operating carryforward not recognized		14,000,000
Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Income tax benefit	2,000,000
Deferred tax assets, valuation allowance	1
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Deferred tax assets, valuation allowance	73,100,000	$ 141,900,000
GENBAND
Operating Loss Carryforwards [Line Items]
Income tax benefit	16,400,000
Deferred tax assets, valuation allowance	10,300,000
IRELAND \| GENBAND
Operating Loss Carryforwards [Line Items]
Deferred tax assets, valuation allowance	6,200,000
Accounting Standards Update 2016-09
Operating Loss Carryforwards [Line Items]
Deferred tax assets, stock-based compensation	$ 700,000

INCOME TAXES - Schedule of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at January 1	$ 8,969	$ 8,888	$ 8,875
Increases related to current year tax positions	139	36	13
Increases related to prior period tax positions	430	723	0
Increases related to business acquisitions	2,012	0	0
Decreases related to prior period tax positions	(7,022)	(81)	0
Settlements	0	(597)	0
Unrecognized tax benefits at December 31	$ 4,528	$ 8,969	$ 8,888

MAJOR CUSTOMERS - (Details) - Customer	12 Months Ended
MAJOR CUSTOMERS - (Details) - Customer	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue \| Verizon Communications Inc.
MAJOR CUSTOMERS
Concentration risk, percentage	17.00%
Revenue \| AT&T Inc.
MAJOR CUSTOMERS
Concentration risk, percentage		12.00%	13.00%
Accounts receivable
MAJOR CUSTOMERS
Concentration risk, percentage	31.00%

GEOGRAPHIC AND OPERATING SEGMENT INFORMATION - Summary of Revenue by Geographic Area as a Percentage of Total Revenue (Details) - Revenue - Geographical area	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue by geographic area and by customer
Percentage of total revenue	100.00%	100.00%	100.00%
United States
Revenue by geographic area and by customer
Percentage of total revenue	66.00%	69.00%	71.00%
Europe, Middle East and Africa
Revenue by geographic area and by customer
Percentage of total revenue	14.00%	13.00%	13.00%
Japan
Revenue by geographic area and by customer
Percentage of total revenue	7.00%	10.00%	10.00%
Other Asia Pacific
Revenue by geographic area and by customer
Percentage of total revenue	7.00%	5.00%	4.00%
Other
Revenue by geographic area and by customer
Percentage of total revenue	6.00%	3.00%	2.00%

GEOGRAPHIC AND OPERATING SEGMENT INFORMATION - Summary by Product and Type of Service (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Service	$ 148,823	$ 106,210	$ 107,121
Maintenance
Revenue from External Customer [Line Items]
Service	114,735	86,995	89,280
Professional services
Revenue from External Customer [Line Items]
Service	$ 34,088	$ 19,215	$ 17,841

RELATED PARTIES (Details) - USD ($)	5 Months Ended	12 Months Ended
RELATED PARTIES (Details) - USD ($)	Jun. 09, 2016	Dec. 31, 2015	Dec. 31, 2017	Oct. 27, 2017
Related Party \| One Source
Related Party Transaction [Line Items]
Revenue from related party	$ 23,000	$ 150,000
GENBAND
Related Party Transaction [Line Items]
Promissory note issued to GENBAND equity holders				$ 22,500,000.0
Minimum \| GENBAND
Related Party Transaction [Line Items]
Promissory note interest rate			7.50%
Maximum \| GENBAND
Related Party Transaction [Line Items]
Promissory note interest rate			10.00%

COMMITMENTS AND CONTINGENCIES - Leases Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Area of leased facility (in sqft) \| ft²	97,500
Rent expense	$ 5,900	$ 4,500	$ 5,200
Years ending December 31,
2018	14,703
2019	11,067
2020	6,632
2021	5,612
2022	3,680
Thereafter	15,866
Operating lease, future minimum payments	$ 57,560

COMMITMENTS AND CONTINGENCIES - Litigation and Contingencies (Details)				3 Months Ended	12 Months Ended
	Oct. 26, 2017 USD ($)	Mar. 17, 2016 USD ($)	Jan. 15, 2016 USD ($)	Dec. 31, 2017 USD ($) cases	Dec. 31, 2017 USD ($) cases
Loss Contingencies [Line Items]
Ongoing lawsuits \| cases				3	3
Damages awarded to other party		$ 0	$ 8,168,000
Damages awarded from other party	$ 315,000
Unfavorable Regulatory Action
Loss Contingencies [Line Items]
Civil penalty				$ 300,000	$ 1,900,000

QUARTERLY RESULTS (UNAUDITED) - (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 146,212	$ 74,629	$ 55,733	$ 53,368	$ 67,572	$ 65,011	$ 60,857	$ 59,151	$ 329,942	$ 252,591	$ 249,034
Cost of revenue	69,413	20,082	19,331	19,620	22,178	21,425	20,629	20,748	128,446	84,980	87,377
Gross profit	76,799	54,547	36,402	33,748	45,394	43,586	40,228	38,403	201,496	167,611	161,657
Income (loss) from operations	(35,663)	3,919	(12,703)	(10,782)	(2,704)	(4,316)	(2,708)	(3,881)	(55,229)	(13,609)	(31,217)
Net loss	$ (15,714)	$ 3,453	$ (12,345)	$ (10,646)	$ (2,631)	$ (3,731)	$ (2,916)	$ (4,654)	$ (35,252)	$ (13,932)	$ (31,895)
Loss per share:
Basic (in dollars per share)	$ (0.18)	$ 0.07	$ (0.25)	$ (0.22)	$ (0.05)	$ (0.08)	$ (0.06)	$ (0.09)	$ (0.60)	$ (0.28)	$ (0.64)
Diluted (in dollars per share)	$ (0.18)	$ 0.07	$ (0.25)	$ (0.22)	$ (0.05)	$ (0.08)	$ (0.06)	$ (0.09)	$ (0.60)	$ (0.28)	$ (0.64)
Shares used in computing loss per share:
Basic (in shares)	86,567	49,753	49,543	49,114	49,232	49,402	49,423	49,484	58,822	49,385	49,560
Diluted (in shares)	86,567	50,131	49,543	49,114	49,232	49,402	49,423	49,484	58,822	49,385	49,560

NATURE OF THE BUSINESS (Narrative) (Details)	Dec. 31, 2017 customer
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of customers	1,000

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting for Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Tax liability related to undistributed foreign earnings	$ 3,171	$ 0

RESTRUCTURING ACCRUAL - Balance Sheet Classification (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Restructuring and Related Activities [Abstract]
Long-term portion of accrued restructuring	$ 0.2	$ 0.1