Not sure... If you're certain of the WiFi coverage, and are still having the problem, I'd start with Apple Care or the local Genius Bar... I would try to construct my own set of experiments to isolate the problem and get more info. I mean the last thing you want to do is to make an appt with the Genius Bar and then have it work as adverized, right? ...You can try a live chat with AT&T support, but something tells me they're gonnna point you back at Apple.
I must say again, though, that net WiFi link would be my major suspicion. I would collect some WiFi connect and transfer stats using a Mac to monitor with say, iStumbler, et al. If that's greek, then find either a local geek, or give up and seek out the Genius Bar.
Good luck.
ADDENDUM: What error message? (...went back and re-read) Pls specify. Have you verified that you don't have a fwd on busy set, and set wrong? That could give an AT&T network message.
Message was edited by: Chas Hulme