ATT Uverse account was closed but not properly. In collections and customer support not helping at all. What do I do?

We closed an ATT Uverse account and returned the equipment, but ATT forgot to shut down the account properly. It eventually went into collections.

Opened a ticket with ATT over a year ago and still it sits as collection on credit report. Spent hours talking to customer support stating "they are taking care of it" but it still never drops off the credit report.

What can we we for this issue?

At this point, I'm about ready to have our attorney draft a letter and prepare for a (Edited per community guidelines).