Abstract: The present invention provides a test set which checks the communication line from a central office to a telephone responder which has a predetermined telephone number and which generates an output signal consisting of a predetermined sequence of signals. The checking is performed from a central testing station from where an operator inputs a test signal which identifies the telephone number of the test set located in the central office from which the test is to be made. Once the connection is established, the telephone number of the responder to be called is transmitted. The test set includes a signal detector which identifies an input signal. The signal detector is interfaced to the responder transmission medium such that the signal detector receives responder output signal as an input signal. The test set further includes a memory which includes the correct signal which should be received from the responder with called telephone number, and a processor.