AMR in Evansville, Indiana is using a smart phone dispatching app and only uses their mobile radio as a backup. A few months ago local news coverage showed AMR dispatch taking an emergency call and forwarding the route and dispatch info to EMR ambulances. All dispatch, pre arrival info and on scene traffic was to be handled by the cell phone app. The mapping program showed the route on the smart phone and other information which would be to sensitive to give over the voice radio system.
Not sure if this app is being used in other cities but might be the reason your not hearing AMR on normal radio channels. In essence this is working just like dispatching being completely done by CAD and MDT (Mobile Data Terminal). With the improved coverage of cell phones many times the cell phone can have more range than two way voice radio.