This is going to be great
I can just imagine GM service departments full of idiots whining that their radio doesn't work and service reps patiently trying to explain to them that they were streaming audio and if there is no cell coverage they are SOL. Or you are driving kids somewhere and the streaming video in the back keeps crapping out, making for a wonderful trip.
On top of that I wonder if the pre-recorded something like this into the car: "The system has detected airbag deployment. Unfortunately since you've traveled outside of AT&T LTE coverage area, this information was not transmitted to OnStar and no help will be dispatched. Thank you for choosing GM and AT&T. If you survive this accident we sincerely hope you recommend us to all your friends"