Sorry to be the one to explain it to you- You've been had.
I spent over six months trying to get MCI/Worldcom to make right, with only about five dollars credit, over that much time, to be shown for it. Less time spent, Certified mail, etc etc.
Many many posts have been made to this site, mostly in 1999 and 2000 about the insurmountable problems ordinary humans have with MCI.
Vote with your dollars and your feet, never deal with MCI again. You will be better off to just resign yourself to it and take the loss.
Yours Truly, Woody