If you made mistake, correct by yourself, att

I received a notice from a collections agency (Bay Area Credit) that said I owe att money. I paid all my bill.I donot owe att money, never. I talked att custom serivce online.He said he can only see one account ( the wireless account) in my ssn.He mean I may have another account that owe money. He said I should ask the account information from Bay Area Credit, then give it to att, let att check if it is right.
I only have the wireless account. And the most ridiculous thing is
If att say I owe money, just show me the proof.
If you said you have proof in Bay Area Credit, you just get it from them and show me.
It is your responsibility to do this, not mine.
Please tell me where I am wrong, or I tell public that you are wrong.