It used to use Verizon's network, which has better coverage than the rest, and has a better antenna. My parents, for example can make a call from their car sitting in their driveway deep in the valley and I have to take my phone part way up the mountain. Then anyone else not on Verizon has to drive 1/2 way to town before they get a signal. Right at their house either my droid or company blackberry are right on the signal/no signal threshold and will eventually get some messages through if you just leave them sit but not quite enough signal to maintain a call so the external Onstar antenna helps there. I wonder if that new Onstar mirror that you can get as an add on has an external antenna or if its all in the mirror so it might not connect as well.