Abstract: A central control unit (scanner) is connected across a set of subscriber loops. At each subscriber's premises, an individual subscriber terminal unit (STU) is connected across that particular subscriber's loop. To each STU are connected the various instrumentalities which are to be monitored by the system, e.g. fire alarms, burglar alarms, etc. FSK modulated signals in the upper part of the audible range are transmitted from the scanner to the STUs at appropriate times. The STUs reply by means of similar signals to indicate the status (e.g. alarm, or non-alarm) of the instrumentalities at the respective subscriber premises. This transmission and retransmission is interrupted when the subscriber's telephone is off-hook. In addition, a signal below the audible range (low tone) is produced at the STU, and transmitted to the scanner over the telephone loop when all the instrumentalities at the particular subscriber premises are in a given state (e.g. non-alarm).