I've got Airvoice in Wyoming. Yes, Airvoice uses AT&T but don't assume it will work everywhere ATT works.
My gf has real AT&T and she gets service in most of Wyoming. I think AT&T has contracts with the local tower operators, and will route your calls on their towers. However, if you have Airvoice, you only get access to AT&T's towers. You won't switch over to Silver Star if the AT&T tower is far away, for example.
With Airvoice, I do not get service in most of Wyoming--there is service in Casper, but not in Pinedale or Rock Springs for example. On my drive through MT, I've gotten service in Missoula and Gardiner, but not in Dillon. I'd experiment with it. But if you live somewhere remote, or absolutely must have service everywhere, Airvoice may not work for you.