Abstract: A telephony network with a digital central office and one or more remotely located digital satellite units. Each digital satellite unit connects to a number of remote subscriber lines and monitors these lines to establish communications through interconnecting spans and a digital satellite interface at the digital central office. The digital satellite interface also connects to port group highways in the digital central switch, to call processors in the digital central switch, and to a maintenance processor in the digital central switch. An automatic test circuit communicates with the interface, and each remote digital satellite unit contains complementary testing circuitry. A test call is made to test the system by activating a test access relay at the remote digital satellite unit and producing a call to the automatic test circuit. The automatic test circuit then generates a tone that is carried as voice data to the remote digital satellite unit test circuit.