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

I received 160.00 bill from a debt collector AFNI for an ATT account in Katy Texas, which I closed out around November 2013. I have had nothing but trouble with billing since day one. I signed up with my Cingular/ATT cell phone number which I have had over 10 years Yet not one call until a debt collector sent me a letter years later.
To begin - I received a flyer in my APT katy, Texas welcome folder about ATT internet service with a direct number to call to setup with the technician. I was also told by the apartment assistant manager named Rob that all the apts were prewired for ATT services up.

I already used ATT cellular for 10 years and I also had a roughly 20 year internet account in Atlanta, GA

I called the ATT number provided on the flyer. I talked directly to this ATT technician and it was a very brief conversation with my information.  I signed on with the exact same ssn# as the other accounts.

I believe someone called to confirm my address and provided a setup date. I informed them that I had the equipment already, they said provide it when the tech gets there.

When the ATT technician arrived at the Apt, my husband directed the technician to the equipment we already had that was sitting on my table. The ATT guy setup the modem/router wireless box. He did not access my personal computer. My husband never signed anything nor did I ever receive a breakdown of the provided services. Unknown to me the ATT technician set up my email address and had all my invoices sent to paperless to that ATT address. So I waited for a bill and then forgot all about it. I finally realized I hadn't been paying a bill and called ATT to inquire about it. I gasped when I was told a huge amount that I accumulated in 2 months. And  the never ending billing issues begin for this account. I asked for a breakdown of the amount. I explain my situation to the ATT rep. It literally was such a mess that ATT had to call me back several times. The phone calls kept commencing even to the hours of a call at 8 pm. I am talking to a male manager with a British accent trying to sort through my bill. He gives me a credit on an internet package that I had not ordered (but I guess had been signed up for by this technician I initially called) and then I believe during all of this back and forth sorting, he signs me up for a better year long service than the original one I never agreed to. I always told them I found the equipment and I supplied it. Every month I had to call because something was wrong with the bill. It was very exasperating. I remember talking to a girl with an Indian accent and I told her I was getting charged for equipment that I had found in the trash. She could see a list of previous notes of me calling. She said ok and saw the notes of me going round and round about the equipment. She saw the notes and went to her manager, but she didn't understand why she wasn't capable of taking the modem bill off, but she could give me a credit for the service charges that was approved by her manager.

My project ended in Houston. I call ATT to properly close out my account. I was told it would be a $45 cut off fee at this point since I was so close to the end of the contract. I paid the cut off fee with my credit card over the phone. I was told I needed to do nothing else. I never received anything else from ATT about this account. Why should I think anything else about this account. I was proactive in closing the account properly and I had 2 other open ATT accounts. One was for over 20 years and the other now has been for 10 years. I always am faithfully paying. And I had I used the same SSN to set up all the accounts under my name.

In 2016 - (3 years after closing the account), I received a notice from Afni credit collector for ATT for a non-paid bill of around $160 dollars. I check my credit in 2016 I have been hit for 60 points after I have been a faithful bill payer all these years. To date my credit went from 820 excellent to 724 good - I had kept my credit pristine for 33 plus years and now ATT has hurt me. I don't understand why I am getting billed for something someone threw away/abandoned and now I am being held liable.  Why didn't the ATT guy say anything when he set it up? Shouldn't I have been told there was a previous bill on this modem? I actually have 2 ATT modems that I found at different times in Apartment trash that I still have in my possession. I guess that last person closed their account that threw the modem away ( I have) will not get charged either. I would like to see a contract that I have signed for renting this equipment I had in my possession before the ATT technician arrived. I asked many times for a paper breakdown of this account. I was given the understanding that everything is verbal and hear say for what you get with AT&T and is it the word of the ATT guy that set this thing up wrong. I never received a document like I sign for service starting and the installation being successful. I know my husband didn't sign anything on an electronic computer.  I never went into the AT&T sight to sign up for anything to start on this account. ATT should have a recording of all my conversations and notes through out my account to prove everything I am stating today. It was clearly written on my account that the equipment was provided by me. I find out about everything when I get a notification through a collections agency bill and a big ding on my credit rating that was 820. I paid all my bills on this account and when the collections guy looked into it he said it was for equipment. He seemed to be willing to find out about my issue. I never heard anything else. I have no clue that a 2 plus year old modem cost $160 furthermore that they are coming after me and not the previous owner of it for abandoning it. I have never seen such a mess in my life. It is sad that I work so hard to keep my credit rating so high to have this happen to me. I did everything in my best efforts to keep the bills straight with ATT. I had issues almost every single month on this bill. I think that should easily be found and hope everything is noted. I am so bewildered and distraught by all of this.