A system and method for verifying the integrity of a DSL circuit on a shared telephone line. The invention uses the amplitude modulation principles to overcome the difficulty associated with a blocking capacitor that isolates a DSLAM on the DSL circuit from the rest of the shared telephone line. A transmitter of the invention mixes a high frequency carrier signal and a low frequency audio signal to produce an amplitude-modulated test signal. The amplitude modulated signal is supplied to the telephone line at a main distributing frame side of the blocking capacitor and is detected by a receiver at the DSLAM side of the blocking capacitor.