Most, but not all, SP stations have a base radio. These radios are essentially the mobile Orion/M7100 in a GE-Ericsson-M/A Com-Harris base radio enclosure. Typically are programmed to mimic the mobile radio channel lineup but is only programmed for the specific zone frequencies needed. So you may hear a station call a car on the "car to car" channel; yeah it's a bit of a misnomer in this respect!
Coverage issues may be resolved by a Troop Control Point calling cars on State frequency if the Troop frequency is either busy, a tower is down, or coverage is spotty. You won't hear a TCP call on "car to car" though because their consoles aren't programmed for it. If you're hearing a TCP on a county or non-SP radio system, chances are it's another base radio setup next to the primary radio console.