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DEBT - Promissory Note (Details) - USD ($)
Apr. 29, 2019
Dec. 31, 2019
Dec. 31, 2018
Oct. 27, 2017
GENBAND        
Debt Instrument [Line Items]        
Promissory note issued to GENBAND equity holders       $ 22,500,000.0
GENBAND | Minimum        
Debt Instrument [Line Items]        
Promissory note interest rate   7.50%    
GENBAND | Maximum        
Debt Instrument [Line Items]        
Promissory note interest rate   10.00%    
Promissory Note        
Debt Instrument [Line Items]        
Promissory note issued to GENBAND equity holders     $ 22,500,000  
Promissory note balance     24,100,000  
Interest converted to principal     $ 1,600,000  
Notes Payable, Other Payables | Promissory Note        
Debt Instrument [Line Items]        
Repayments of outstanding amounts $ 24,700,000