The nearest place to get signal is probably on South Fork Road just off of 168. If you look you'll see a turn-out along the shoulder of the road where lots of people pull-over to make phone calls. It's between Forks and Four Jeffreys campgrounds. I think it works because it lines up down the canyon with Bishop. I'm not sure about A.T.&T. but most of my friends use their cell phones at that spot.
AT&T no way. Even Verizon doesn't get reception up there. Last weekend a Verizon signal kicked in for a moment just before we turned onto 168 but then dropped again until around the Buttermilks turn-off.