AT&T Error in billing. Sent to collection. Dealing with this since Nov 2014 Nationwide

We have had internet service with AT&T for going on 25 years. We moved our services to a new home address on November 6, 2014. We received some past due bills over the next several months. I contacted them multiple times. They stated that when we moved, they transferred the service to the new address, but they had a computer glitch that did not close the first account. This was discovered in May 2015. They stated that hen I made my payments, sometimes it was applied to the new account and sometimes to the old account. They stated they were closing the old account effective Novermber 6, 2014 and returning any payments that had been applied to that account. In about 6 weeks, I received another bill on this account. I called them again and spent hours on the phone going over everything again. Supposedly it was taken care of. This has happened about four different times. Then last month, August 2015, I had to call them yet again about this. I was on the phone for about 3 hours. They stated this was completely taken care of, again, and I would never hear from them again regarding that account. They stated they would send me in writing the account status as being closed with zero balance. I have yet to receive that letter. Instead I receive a letter from a collection agency about this bill. I contacted AT&T yet again, to be told that there is still a balance on this account,  the customer service department was closed. 

To to say I am at the end of my rope would be putting it mildly. This is so unfair that they can continuously bill me for their error and I have to spend hours on the phone trying to defend myself and sort it out time  and time again. I need to know what I can do to have this issue permentaly resolved and have AT&T make some type of restitution for the time and harressment.