Here is a bit more information on security systems, from Richard Fitzgerald in Tulsa, OK. Richard is not a contributor to RCM, but I "drew" on him for more information.
Can't remember the name, matter-of-fact I just threw it out during the great dumpster filling of 2003. Most of the current burgler alarm equipment is made by ADEMCO. Most of the home security companies just have their name stuck on it. Most modern systems are modem based and dial into the monitoring centers computer and dumps a numerical code identifying the location and type of alarm. Some of the better alarms monitor the phone line for -48V and in the event they loose the voltage they will dial in via a cellphone type of connection. Used to most of the central staiton monitoring was done over a leased pair with a 10ma. current loop, the monitor panel had a under current and over current relay for status. You probably remember those in a lot of the small town police stations usually a small panel with several meters with a couple of lights, it was made by ADEMCO. In larger towns (Tulsa) they would use a series loop that would take in as many as 20 businesses in a large series loop called a McCullah loop. At the central station they had a pen register that moved in relation to a break in the circuit. Each business had a closed loop for the alarm and a relay with a code wheel that was in series with the alarm loop. If you had a premises trip it would cycle the code wheel and the cestral station would mark the code of opens and closes on a pull paper looked a lot like tty paper, the operator would read the code and dispatch to the location bases upon the code. Digital telco CO and interoffice fiber trunks has did away with these type s of alarms