A test system for diagnosing twisted pair wiring includes a system processor in communication with a subscriber line interface module, which includes a codec and a SLIC (Subscriber Line Interface Circuit). The SLIC transmits a waveform having a frequency used for voice or baseband modem transmissions onto the twisted pair wiring. The system processor transmits a digital PCM (Pulse Code Modulation) stream to the codec, and the codec converts the digital PCM stream into the waveform transmitted onto the twisted pair wiring. In response to the transmitted waveform, the SLIC senses current on the twisted pair wiring. The codec generates an output waveform corresponding to the sensed current. The system processor receives the output waveform from the codec as a digital PCM stream, synchronizes the transmitted waveform and the output waveform at a single sample, and determines electrical properties of the twisted pair wiring from the output waveform.