mekender-
You can try calling the CRA directly. They may be able to give you more information. If they have validated in the past, then there must be valid contact information on file at the CRA for them to correspond with. Disputing it wasn't be the best course of action, since you know it is a valid debt, and that is all disputing does: gives the company the chance to verify whether the debt is valid or not. Yes, technically, the CA is supposed to list current and correct contact info on your CR, but taking due course would probably involve you attempting to find out anything from the CRA as well.
Otherwise, you can always try to contact AT&T directly. If it is with a CA, they should have the CA contact info on your file.