An apparatus for testing the operation of a digital speech interpolator provided in a transmitting station and a receiving station in a time division multiple access scheme (DSI/TDMA). The apparatus comprises a transmitting unit and a receiving unit. In the transmitting unit on the transmitting station, a pseudo random code for each one of a plurality of channel is generated, a channel identifying signal is generated, a fixed signal having a level lowerthan a detectable level of a tone detector of the digital speech interpolator (DSI) on the transmitting station is generated, and a pseudo tone burst signal for each channel is generated. The pseudo random code. the channel identifying signal and the fixed signal are used under the control of the pseudo tone burst signal, for generating a time division multiplex signal. The time division multiplex signal is supplied to the DSI on the transmitting station. In the receiving unit on the receiving station, the signal sent from the DSI on the transmitting station is received for detecting the pseudo random code. the channel identifying signal and the fixed signal.