Hopefully AT&T will get this taken care of pronto.
In the meantime, send the CA a DV as suggested.
You said you received a collection, was it reported or did you just get the letter? If you just got the letter, then they can not report after getting the DV, unless they validate. I would get the letter sent CMRRR ASAP.
I would stay on AT & T until they fax you something showing everything is taken care of. Get the rep's name you dealt with and is handling this. Call every day or every other day if needed.