I saw folks succeeding with telephone calls last week in the parking lot using AT&T. I didn't try telephone calls, and my phone didn't show service the only time I looked (AT&T also), but I sent an email (when I did not appear to have service) and it did go through within a few minutes from when I sent it.
So, viz AT&T, I would say sporadic voice service, with emails/texts going through, if not instantly, then quickly.
If you drive East around the ridge so that Lone Pine is in sight, I bet you can get more reliable service....
