We had AT&T until we started going each year to a park in southern Colorado and could not get service at the park even though some other people with AT&T and different phones could. AT&T suggested trying different model AT&T phones. No luck with a different model phone either. And they would not allow us to return those phones that would not work. Said we could buy additional models and try again. Could they guarantee another model would work? They said no. Then they basically brushed us off after that saying they did not service that area. Really? When I asked about other people were using AT&T in the park, they had no answer except repeating that they do not service that area.
Changed to Verizon and could get service there reliably afterward. And have been very happy with it everywhere else we have gone.