Well it was odd as there were 4 people in the vehicle, 3 different cell providers between all of us and nobody had service but Onstar worked!
That's another small advantage most people don't have these days when traveling. With OnStar, you are using a 3 watt transceiver with an external antenna. Whereas with your portable phone, it is limited to a maximum of .6 watts. Put that portable phone in a metal cage along with possibly having a metallic based window tint applied, and then drop yourself in a remote area, there's a good chance you won't call out on that little phone.