Actually I would call Apple support first.
The AT&T account is set up with your Apple ID by default and uses the credit card registered with them. If you haven't updated your credit card info with Apple that may be it. Or it maybe they have to manually change something with the way they work through AT&T.
But either support line is your best first move. If you get the wrong one they should at least know enough to tell you to call the other. Just pray you don't get a round of telephone support ping pong.