U Verse wrongly sent me to a collection agency. I have not been a customer since January, 2012.

I recently received a notification on my credit report that said U verse has sent a bill to a collection agency in the amount of $384.  I have not been a customer since January 2012.  I returned all equipment and did not have an outstanding balance when I ended my service in January 2012. I would appreciate some information as to the reasoning for sending me to a collection agency.  This has affected my credit score by lowering it by 41 points.

Please advise on how AT&T is going to correct this mistake.