In the movie, Parker (girlfriend) had a phone in the locker but was talked out of getting it by the boyfriend as he tried to repair a wrongly taken conversation.

AT&T in Arizona is horrible outside of town and that includes stretches of state routes between cities, (not major freeways). I have been on the top of 4 Peaks and at the 1.5 mile rest house in the Grand Canyon and AT&T had no service. My hiking partner who has Verizon had good signal at both and had to use his phone. In the Superstitions when I had Sprint I knew exactly where I could pick up service coming out of the canyon. With AT&T I have to travel an extra 4 miles out and even then have to reset my phone or wait another few minutes to pick up a tower. YMMV with provider and location.