Alarm systems typically incorporate a digital cellular communication unit (or communicator) that will contact the central station (or some other location) via the Public Switched Telephone Network (PSTN) and raise the alarm, either with a synthesized voice or increasingly via an encoded message string that the central station decodes.

A dual signalling system would raise the alarm wirelessly via a radio path (GPRS/GSM) or cellular path using the phone line or broadband line as a back-up overcoming any compromise to the phone line.