the test system of the invention uses suppressed ringing connections from a test unit to the customer premise equipment via the central office to establish connections for performing the loop - back test . the suppressed ringing connections are established over standard voice trunks between a switch in the central office and the test unit by a central office service unit such that the number of lines that can be tested simultaneously is limited only by the number of trunks linking the cosu to a switch . because these trunks are relatively inexpensive and simple to install and maintain , the test system of the invention can be readily installed and expanded in the existing network to provide virtually any level of monitoring capability .