MCI review: Fraudulent bill and unethical behavior

On Dec 4 th I moved from LA to Napa. I cancelled my international dialing phone service with MCI on January 5, 2017 as they no longer offered the same deal I had in LA so decided to move my entire account to AT&T. I ordered a new service with AT&T for which I got a confirmation code.
Then, when I tried to make a call to the UK i could not so waited another week or so until the line was connected, which I assumed was with AT&T.
I then get a bill from MCI for over $666 - when I called and said this was a mistake I find out that AT&T had not connected the int dialing so MCI continued to charge me at a regular (excessive)rate. This is unconscionable. When I called and talked to Wendy Duffie in their Executive Office she was rude, short, kept repeating the same information over and over again and hung up on me. Now, it is March 14th and after trying to sort this out with both AT&T and MCI I demanded a letter about this. The letter is saying that I approved the additional billing by pressing 1 when I dialled internationally. This is a blatant lie. I did nothing of the kind. Now when I call Ms. Duffie is just rude and short. Not only is this unethical but MCI is lying about what they said or I did.