This could be for a variety of issues. First, all of the units receive their call information over a cellular-based system called Logis. Logis automatically assigns units to a call based on predicted fastest unit on-scene. As it is automatic and does not involve the dispatcher for assignment, many times a dispatcher has to override the assignment and re-assign the call to another unit. For most areas, AMR dispatch is NOT the 911 call-taker center, but they are linked. With this link, if there are updates by the 911 operator, these updates automatically are sent over Logis to the responding units. This takes away much of the need to talk over the radio. Logis also has a built-in text service, which allows some communication between crews and dispatch, again diminishing the need for talking over the radio. Lastly, there is a side program that also functions as a push-to-talk voice network on the same cellular device called ESChat. I personally hate ESChat and would much rather use a traditional radio. In an emergency, you can't entirely rely on cellular networks, managed by someone else, meaning having a radio backup is a good idea. Many people have become too reliant on cellular without questioning, "What if that fails?"
AMR I believe has maintained an old analog radio channel in Placer County. In Yolo County, they had a backbone failure YEARS ago and never fixed it until a couple of months ago. The old Yolo County system involved a radio voter system with several remote sites with an old, dedicated phone line constantly connected to the voter in Sacramento. The voter system required a response from each site and if one site went down, the entire system would go down... STUPID design! As phone companies have been moving lines around, they would often disconnect this always connected line, not thinking it would matter. They finally replaced the system with a repeater on Mount Vaca.
I'm not sure if they have done so in Placer County, but it would not surprise me if they've gone the way Yolo County has gone. In Yolo County, AMR has gone to DMR; this is known in the Motorola world as MotoTRBO. This technology is TDMA, which makes it possible for two different conversations to be taking place on one frequency pair. I do not believe there is any encryption being used, so if you have a radio capable of DMR/MotoTRBO, you should be able to listen. In Yolo County, one time-slot is used for primary communication while the other is for tactical.
