I think it is the Iphone itself, not ATT. I have the Iphone and it is great for everything except for being a phone. It drops calls quite a bit and some places it is hard to get a signal. DH has ATT, but not the Iphone. Pretty much everytime I don't have a signal, he still has one on his phone and can make calls easily. Also, he never has dropped calls with his phone.
yeah, if that doesn't fix it, i'd ask to try an iphone 3gs. if that doesn't do it then i'd drop it. if you have to change out the phone i'd document it and ask that you want another thirty days to give this a try.