Brent - I've hiked out of Horseshoe Meadows dozens of times, and used to have AT&T and now Verizon. The last reliable spot seems to be the turn where the hang gliders jump off. Beyond that, it would be rare to get a signal. Even if you're in line of sight with the Owens Valley higher up, don't depend upon a connection - for most cell phones it's simply too far away.