Abstract: A "call-for-service" and continuity sensor circuit for use with a trunk circuit of a digital PABX telephone system interfacing with a central office via ground start signaling providing for detection of a "call-for-service" signal to the PABX by the central office. This circuit also provides high and closely matched tip to ground and ring to ground impedances for all trunk conditions in order to prevent excess loading of the trunk circuit. This circuit provides a precise and stable threshold detection level necessary for accurate sensing of the trunk circuit conditions mentioned above despite parasitic leakage current, induced longitudinal power line voltages, and earth potentials sometimes found to exist on the line connecting the central office to the PABX.