How do I reverse - ATT UVERSE and AFNI debt collectors ruining my credit

In September 2015 I closed my ATT Uverse account in Florida and moved to California where I re-contracted Uverse. Upon signing up, I was told that I had no outstanding debt with ATT. In May 2017 I received a letter from a debt collector looking for $92 for an outstanding ATT bill. I brought this to the attention of ATT who once again confirmed that I had no outstanding debt with ATT. This week I applied for a mortgage, seeing a credit report from Equifax that my credit was over 800! To my surprise I was rejected on the mortgage because of an outstanding ATT debt collection letter for the debt that ATT had told me did not exist. Once again I contacted ATT who once again confirmed that I had no debt with the company. After having them search for me by SSN, they finally discovered this $92 from over 2 years ago that had been sent to debt collection, despite me being a current ATT customer and despite ATT´s repeated denial of the existance of this debt. And ironically, part of this debt being charged is from after I closed the account, returned the equipment to ATT, and even cut the utilities at the Florida address!

Now I have spoken with 25 different ATT representatives for over 6 hours in the past two days and no one seems to be able to help. Not only has ATT billed me for services after I returned the equipment to them and called them to cancel services, but now they have sent my name to debt collection, causing a 150 point hit on my credit score and making it impossible for me to close on my mortgage, despite me being a current customer.

Why would ATT simply not bill me for the debt, rather than sending me to collection? My SSN is the same on both accounts, and on multiple occasions ATT told me that I had no outstanding debt, even when I asked about the debt collection letter a year ago.

I would pay this just to be done with it, but unfortunately it would still appear on my credit history that I had an outstanding debt for over two years, despite ATT´s assurance that this debt did not exist.

ATT should expunge this debt from my credit history in all three bureaus because:

1. I should have been contacted directly about the debt (although my address changed, my email and cell phones still worked)

2. This debt should have been passed on to my new account in California

3. ATT denied the existence of the debt on multiple occasions, including when I received a letter from their debt collection company

4. Part of the original debt was charged erroneously after I had ended services with ATT and returned the equipment.

Office of the President, please assist this loyal ATT customer who soon will be changing residence and will be making a decision about cable and internet services for his new home. Your call center employees seems unable and unwilling to help.