Account went to collections

I would like to know who I contact in order to give confirmation that I have paid a past due account that went to collections. How do I get confirmation that my account has been documented as paid. I have confirmation # from collection agency. When I called AT&T they refuse to give me any information, claiming there is a passcode. I have talked to several billing specialists about this account and never once had them tell me my passcode is incorrect. I am so frustrated right now I could scream. I held for 28 minutes for 2 different reps for them to tell me I need a passcode. The account is closed. It's in collections, there is no passcode to my knowledge. And the last 2 cust serv reps this evening have been horrible. Rude and unhelpful. They act as if I'm stupid. Please help me figure out who I can call at AT&T to give confirmation of account paid in full?