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MAJOR CUSTOMERS
12 Months Ended
Dec. 31, 2019
Risks and Uncertainties [Abstract]  
MAJOR CUSTOMERS
MAJOR CUSTOMERS

The following customers contributed 10% or more of the Company's revenue in at least one of the years ended December 31, 2019, 2018 and 2017:
 
Year ended December 31,
 
2019
 
2018
 
2017
Verizon Communications Inc.
17%
 
17%
 
17%
AT&T Inc.
12%
 
*
 
*

* Less than 10% of total revenue.

At December 31, 2019, one customer accounted for 10% or more of the Company's accounts receivable balance, representing approximately 22% of total accounts receivable. At December 31, 2018, two customers accounted for 10% or more of the Company's accounts receivable balance, representing approximately 32% in the aggregate of total accounts receivable. 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.