Probably the only places I hike in the Sierra with any kind of reception are in the Tahoe basin and off the Interstate 80 corridor. Anywhere else, nada.

Calistoga, in the wilds of northern Napa Valley, probably offers fine reception when hiking the hills. The Oat Mine Trail starts at the lower left corner of this sat photo and ascends the ridge via an old road as seen in the overlay. It's a nice wintertime trip, just mind the ticks.

Oat Mine Trail

Cheers,