When I'm in my car my S20 connects by Bluetooth and I can made and receive calls OK, but I can't do anything on the phone itself that needs a connection to the network, such as bring up the keypad to respond to a "Press so-and-so" menu or use Google Maps to identify my location. This has been a nuisance several times but was a serious problem a few days ago when I needed roadside assistance and they wanted to know the postcode of my location. The phone only starts working normally if I switch off the ignition and take the phone some distance away from the car. I think it was not like this originally but has changed.
