i think there is actually a SIM card, or whatever the auto equivalent is, in the car itself. My BMW assist functions (google, connection to bmw customer service center which shares my location etc with bmw, google maps to car, etc) all work just fine even if my cell phone isn't with me, or if bluetooth is off and the phone isn't paired with the car.
Which leads me to think that the car has its own comms facilities for the assist functions.
I have contacts within a BMW dealership that I can quiz for more specific details if that helps
many thanks and all the best