An alarm system is disclosed which utilizes the Bell System DATAPHONE (a registered service mark of AT&T Company) Select-A-Station Service equipment to communicate alarm messages from a plurality of protected premises to a central monitoring station. The system includes a plurality of transponder units, one at each of the protected premises, connected to the central monitoring station through the Bell System equipment. The central monitoring station sequentially polls all the transponders. This is accomplished by sequentially setting up connections from the central monitoring station to the transponder units and sending a START tone burst. In response to receipt of a START tone burst, the connected transponder transmits a tone burst reporting the status of the protected premises. This tone burst contains six tone-pairs in defined time slots, each of the tone-pairs identifying the condition of an associated protected zone, AC power status, and whether the protected premises is in its DAY or NIGHT mode. An alarm condition causes the absence of a tone-pair associated with the particular protected zone. The central monitoring station monitors for the absence of tone-pairs and if two out of three consecutive polls, in their proper turn, of the transponder at a particular protected premises reveal the persistence of an alarm condition, operating personnel at the central monitoring station are alerted so that the proper authorities may in turn be notified. When a DAY to NIGHT transition is requested by a customer at a protected premises, the central monitoring station transmits a RINGBACK tone burst, which is the same frequency signal as utilized for the START tone burst, except that it occurs during a different time interval of the scan time. The central monitoring station includes a memory for storing the status of each of the protected premises, a display, a printer, and associated control circuitry.