Since Verizon's LTE network is available in over 100 cities, I think that it would be a selling point for AT&T if their LTE devices were able to roam on other networks for faster data transfers. I am reasonably certain that AT&T has voice coverage in the urban markets currently covered by LTE.
There are probably several reasons why this is not the case, but I doubt lack of voice coverage is one of them. AT&T and T-Mobile both use GSM frequencies, but it is reasonably difficult to find phones that use the four frequency bands that they use for 3G coverage.
There are probably several reasons why this is not the case, but I doubt lack of voice coverage is one of them. AT&T and T-Mobile both use GSM frequencies, but it is reasonably difficult to find phones that use the four frequency bands that they use for 3G coverage.