Abstract: The inventive monitor system uses a monitor circuit 15, 25, 45 connected to a telephone circuit 10 and containing an electric switch 20 that changes state in response to change in a monitored condition. A capacitor 16 and high pass filter effectively isolate the monitor circuit from the phone circuit so as not to interfere with use of the phone circuit, and the monitor circuit is energized by a small portion of an ac ringing signal in the telephone circuit. The monitor circuit interrupts ringing of the phone circuit by briefly interacting with the phone circuit to electrically simulate answering the phone circuit. The number of rings before interruption conveys information about the state of the switch and the monitored condition when the phone is rung. The ringing interruption can occur after different intervals to indicate different circumstances, and several switches 20a and 20b can be used for monitoring several conditions.