Abstract: A telephone subscriber interface device provides privacy on a party line. The device provides for revertive calling, break-in, automatic disconnect, selective ringing, automatic number identification, and operates on the central office power supply. The device provides the following features: A connect state for the PLC in which the PLC is connected to the telephone line but in which the PLC is not responsive to other inputs, a ring decoder which detects either divided coded ringing signals or divided ringing signals in a novel manner, means for testing whether a revertive call has been properly terminated, a low power switch so that the PLC can run on telephone line power only without battery backup, and means for resetting the PLC to a ready state in the absence of other inputs.