Lots of places, at least out here in the west, that there is no signal. In fact, many times I lose the signal while still in the car driving to the trailhead. And the phone quickly uses up its battery when searching for non-existent signals. You're more apt to get a signal when up on a ridge, but that won't help if the emergency happens down in a canyon. IMHO, I'd never want to rely on a cell phone for any kind of emergency coverage when hiking or backpacking.