We have great AT&T coverage in Texas. That's why. My best friend in LA, she has problems w/ voice in some areas, others not at all.
I've use Sprint, T-Mobile, Verizon and AT&T and have friends who use all of them. Universally, Sprint is terrible. Everyone else reports equal problems with the other 3 at various times. It's just an ad war. AT&T has less coverage penetration, especially in rural areas, but they have the phone everyone wants. Verizon has decent phones but not THE hot phone, but better coverage penetration. So they're going to scuffle. AT&T's complaint was that the Verizon map seemed to indicate that the areas represented by white, AT&T had no data coverage. But they have decent EDGE in those areas. They have a point. Had Verizon used some other color and noted it was EDGE-only for AT&T's weak spots then this wouldn't have come up, but then it wouldn't be a proper ad, would it? They're not going to tell you the clear, credible truth, not either company.