I can't be specific but there is a section of trail around the Carlisle mine that has 5 bars for Verizon. Text and voice. Another spot I can recommend but is a hike is the top of the granite hill on the uptrail side of water crossing #1. Quite a hike but works if you really have to get a text out. Not voice.
I agree with the comment about Meadow. Go up near the dam, stand on the big rock, on one foot with your other hand in the air facing NE and don't breath and you'll get reception...maybe.
California 4 Wheel Drive Association
American Motorcyclist Association