Huge ATT Billing Error: $1,200 landline bill

I'm a longtime (20-year) AT&T customer. I have landline and DSL service, which typically costs about $85/month. I've had the All Distance plan for about 15 years and would never change that. I've never been late paying a bill.

From June through October, my DSL went out several times, sometimes for several days. Then in September, my bill shot up by about $10/month. Annoyed that I was paying more for spotty internet, I called and asked if the bill could be lowered back to where it had been. The person I spoke to was very nice and offered to lower my bill. She assured me I'd be paying about the same as I always had for the same service. I suspect the mess that followed came from her mistake.

My October, November, and December bills were normal. Then came my January bill — which was almost $800! Thinking this was some crazy mistake that would be easily fixed, I immediately called AT&T. The person I spoke to — her name is Tina, and she works in the Houston office — promised that she would fix this mess. She said the "All Distance" plan had been removed from my service in September, which means I'd be paying for all long distance calls (and apparently calls to a neighboring town 15 miles away is "long distance").

I NEVER requested this change, nor would I have agreed to it, nor was I notified of it. I would never, ever not have All Distance service. 

Tina put the All Distance plan back on my phone, and she told me to pay $86 for my January bill, which I did. A couple days later, I called again, just to confirm that the problem was fixed. This time I spoke to Dan, in the Kansas office. He assured me that everything would be fixed, and my February bill would reflect the corrections. He confirmed that I should pay $86 for my January bill.

I just received my February bill. It's for $1,200!

I called again and spoke to Robert in the Kansas office. He spent a lot of time fixing the new errors and told me to pay $115 for my February bill. He assured me that someone would call to let me know the matter has been resolved.

Well, when I tried to pay $115 online,  AT&T refused my payment, saying I had to pay at least $660. Also, I never received a phone call.

It's been more than a month, and not only has this problem not been fixed, but it's worsening.

I'm very ready to cancel AT&T completely and get my internet and home phone service elsewhere, but I want this matter to be resolved. This is AT&T's error, and the company needs to fix it. Any ideas for whom I should call next? The FCC?