A security central station monitors a plurality of individual subscriber security systems installed in residential or commercial accounts. Each security system at an account is programmed to periodically place a telephone call to the security central station receiver, and then to send a signal to the security central station receiver over the placed telephone call. The ten digit telephone number of the account is programmed into the security central station as the ten digit account number of record. When the signal sent by the security system is received by the security central station receiver, the security central station compares the ten digit caller telephone number with the ten digit account number of record. Normally, the ten digit caller telephone number will match the ten digit account number. However, a mismatch between the two ten digit numbers indicates to the security central station that the telephone number of the account premises protected by the security system has been changed, and alerts the security central station to investigate the matter further.