Apparatus for use in a signalling system for communicating with a centralized communication center over a telephone network. The signalling system includes apparatus operative under prescribed circumstances to produce an alarm condition for initiating an alarm communication by which alarm data can be communicated to the centralized communication center over the telephone network, and apparatus operative at a prescribed time of day to produce a self-check condition for initiating a self-check communication by which prescribed data can be communicated to the centralized communications center over the telephone network. The apparatus in accordance with the invention includes a programmable read only memory which stores telephone number data representing first and second telephone numbers, and a microprocessor which utilizes this data in attempting communication with the centralized communications center. In accordance with the invention, the microprocessor uses the first and second telephone number data for alarm communications, with the first telephone number data being used first, for up to a predetermined maximum number of times, then the second telephone number data for up to the maximum number of times, then back to the first telephone number data, etc. For self-check communications, only the first telephone number data is used, this telephone number data being used continuously for an indefinite number of times.