Faults or degradation of performance of a voice/data communication network (2) are identified without the need to dispatch service personnel by selectively testing isolated sections of the communication network. For some of these tests, the communication network address of a customer's modem under test (106, 114, 60, 66) is retrieved from a database (88) for use in directing these tests to the customer's modem (106, 114, 60, 66) and for emulating the customer's modem (106, 114, 60, 66) during one or more of the tests.