Abstract: The invention provides a cost-effective system and method for testing telephone lines connected to transmission systems, such as Subscriber Line Multiplexer systems. The invention uses the same pair of wires for transmission and test. According to one embodiment of the invention, a system for allowing a test procedure to be performed on a communication system is provided. The communication system has a transmission line for connecting to telephone exchange and a subscriber line for connecting to a communication set. In the system of the invention, a detector is coupled to the transmission line for detecting test signals on the transmission line. A bypass circuit is coupled between the transmission and subscriber lines for allowing the test signals to bypass the communication system. Also, a discriminator is coupled to the subscriber line for discriminating a connection status of the communication set.