We use and not only the Cheif's but the firefighters as well love it and swear by it. We are a combination department, it is used for those calls that don't go over the radio. i.e. LEO need an Engine to standby while the SRT team performs a high risk entry. We use it additionally so the on duty Captain knows what volunteers are available for calls that day.
All members have a radio but the radio traffic is excessive, you all know what I'm talking about. A structure fire comes in, which for us is an all call, both volunteers and off duty paid alike. The responding units need the radio clear for additional information and first arriving scene size up. My last department everyone had a radio and there were a number of times it would take me numerous attempts to give my arriving scene size up simply coz everyone with a radio was keying up going enroute, asking for directions/address again etc.
This system removes all of that, volunteers and off duty personnel simply speed dial the number on their way to the car or whatever and either press 1 number or do nothing and the line will disconnect and automatically place you as responding. The first arriving off duty Officer at station 1 checks a TV monitor in the bay to see who is responding to the station.
Great system, fairly inexpensive and a great tool at your disposal.