AT&T is currently doing the same thing BT did to you. I closed my account and shipped back hardware a year ago. Since then, they have been reporting either a bill with a random number from $100-3,000 or 0, and being either on time or late or none. It's maddening.
I'll described what I had to do to fix the BT nonsense, in case it's useful for your AT&T problem.
To fix the BT nonsense (after I'd done tedious detective work to figure out which company it was, as it was under a company name I didn't recognise), I was lucky to get a good BT customer service agent, who explained that they could see it was BT, that it happened to other people as well, and the BT Equifax-handling department would fix it as soon as they looked at it, but that department wouldn't fix it on my request. To fix it I needed to complain to Equifax myself as a data correction. Equifax could not change data, since it merely aggregates data supplied by other companies and is not responsible for the data, but it would contact the BT Equifax-handling department, who would make the correction and submit it back to Equifax. In the end getting Equifax to process a data correction complaint was excessively difficult, slow and intrusive (requiring various documents I didn't care to send them), and I found a third company who magically made Equifax process the complaint in days rather than months; it went all the way along the chain to BT and back, and was magically fixed.