Not sure how you'd have an account that old, but anyways...
I agree that, based on what you've said, it should have been enough. Two things, one, you might consider getting your credit score from FICO (myfico.com) and just make sure it's still decent. Secondly, you may ask AT&T why they're denying you credit. Normally I'd say that they have to tell you, but I'm not sure if a subsidized phone strictly counts as credit or not.
Or get your free credit report at annualcreditreport.com (NOT free credit report, those scum). There won't be a score in there (you have to pay for that from FICO) but it's a quick way of checking all 3 sources of credit info.
You do have to check all 3, AT&T pulls from one of those 3, and the information between the 3 isn't totally sync'd up. You could have 2 great reports and think you're OK when the 3rd has red flags on it.