How do I file a formal complaint for billing dispute with AT&T

I have been with AT&T service for over 10 years. We were one of the charter U-Verse customers in our area when U-Verse launched. Until recently, we had never had serious issues with our service. I always made sure to call in prior to promotion expiration dates and they always worked with me on renewing my promotion or finding the best option for me. I called prior to the end of Dec billing cycle to have my promotion extended. The rep provided best pricing for U-Verse and Internet, but it included a phone line. I told him we did not have a need for a phone line. He stated that it would be ok - we didn't need to activate and could just take advantage of the new pricing. Great. Flash forward to January. I received my new bill and it was almost $65 more than what I had been quoted. I called in and spoke with the retention lead and basically, the prior rep added the changes, but they would not go into effect until the Feb cycle. This did not make sense to me. Why did I have to wait a month when I followed their practice and alerts to my account online. The retention lead adjusted my billing and credited my account by @ $63. A couple of days later - service issues come full storm. My husband and I work from home and could not get our IP phones to work, Internet was up and down or not working at all, we couldn't even get on wireless with our phones. Days of this and MANY irritating conversations with AT&T (poor service - hung up on - multiple transfers - hours with Technical Help Desk - etc), they determined that that when the rep placed the order with the phone bundle (even though we weren't going to use it) they moved our facilities that resulted in the outages. A REALLY NICE retention lead helped again. He had the facilities corrected, got our service back to correct service and facilities and we were working again on 1/11. They also apologized for the outage (2 days missed work on my part) and were issuing a $50 credit (this has yet to be applied to my account). All the while this was going, in our frustration my husband and I starting looking at Xfinity and Sling TV. We were late adopters to the  new technology but wanted a back up plan in case AT&T couldn't figure it out. We had to have internet service to work from home. We placed an order with Xfinity. When AT&T corrected the issues, we debated cancelling the Xfinity order. We opted to switch. We couldn't risk it. We had Xfinity installed on 1/14 and OFFICIALLY DISCONNECTED our services with AT&T on 1/15. They assured us the billing would stop as of that date, that our services would be disconnected by 7pm and we were officially gone. Do I miss U-Verse? Yes. Am I getting used to Sling? Yes. Do I love the bandwidth service with Xfinity? YES! Flash forward again. I call today to get my final bill updates. I did not pay my January bill because I was told I would be issued a $50 credit (above) and that my billing would credit back to 1/15, my disconnect date (January bill was for 1/7 - 2/6). So far, I have spent almost 45 minutes on chat with "Hernando" who informed me he had to "make my math" - whatever that means. It took me 20 minutes in wait time just to get to him. I have called the 800-288-2020 line 3 times - hung up on the first time, transferred - disconnected the second time, and third time had an encounter with Miss Kia in the Charlotte, NC call center that was obviously prescripted FAQ and robotic. She informed me that effective January 1, ATT would no longer prorate. That I would have to pay through Feb 6, even though I disconnected. I went through every notice on my online account and look at billing messages for past 6 months - she informed me that I should have received an email and she would send it again, if I could give her my email. I have yet to receive it. I asked for a supervisor or lead. She said her supervisor was out for a month and that all the other supervisors must be in a meeting because she didn't see one. Seriously? Like they have no escalation path? (She said no). We tried Corporate Executive Complaint numbers - they couldn't provide us with anything and kept saying they did not offer support. People. I worked for a Telephone company for 20 years. My parents as well for 30+ (Support Center and Central Office). Part of time in recent years was spent managing triple play offerings, regulatory affairs - rate increases, billing changes and customer notifications, along with building a retention team and managing call center support. I know what I am talking about. I did not receive a notice. I have NEVER in my life been treated as I have by AT&T. Not even on my worst day or worst customer experience. What do I do now? I just contacted a friend that lobbies for AT&T, CenturyLink and other Telcos. I am filing a formal FCC and PUC complaint. I will not pay for services I did not receive. I want my $50 credit, I want my bill adjusted back to 1/15, and I will pay for the service from 1/7 to 1/15. Nothing more. I have rights as a consumer, there billing change was not communicated to me and is a Regulatory requirement and I refuse to be belittled and treated like my service and prior loyalty do not matter. THEY and their SERVICE is why I left after 10+ years and they don't care. They will one day when reps are downsized by automation and increased churn activity. Oh and we have Digital Life with them. Guess what we will be disconnecting next.