iPhone works fine with car bluetooth for Playing Music but not working for Phone Calls
Buried in the last couple of updates has been a little bug that does this. I think it is tied in with the odd MAC address changing Apple devices can do for security and hiding you from tracking.
The only fix I have found is to delete the pairing on both sides (phone and in-car) and then recreate it. You may get away with just deleying the connection on the Apple but I started in the car system first :-(
Solved exactly the same issue in my Renault after about a month of the phone randomly needing reconnecting even outside a call.