Abstract: A plurality of smoke (or heat) detector-transmitter units (17, 19) and/or a plurality of intrusion detector-transmitter units (21) are mounted at various positions throughout a premises to be protected (11a, 11b, 11c . . . ). The units are wireless and transmit a signal whose code relates to whether the unit is a smoke (or heat) detector-transmitter unit (17, 19) or an intrusion detector-transmitter unit (21). Also located on the protected premises (11a, 11b, 11c . . . ) is a receiver-alarm unit (23) that receives and decodes the signals produced by the smoke (or heat) detector-transmitter and intrusion detector-transmitter units (17, 19, 21). Upon receipt of a coded signal, the receiver-alarm unit (23) automatically seizes the telephone line (13a, 13b, 13c . . . ) running from the protected premises (11a, 11b, 11c . . . ) to a central location (15) and produces an "operator" dial signal.