Abstract: 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.