MCI Worldcom Wireless - continues billing me 9 months after I cancelled service ripoff scam

On May 31, 2001, I faxed a letter to MCI Worldcom Wireless to cancel my cell phone service. I had waited to cancel until my contract was up so I would not have to pay their penalty.
When I received a bill in June, I was told it was for the period of service before I cancelled the service. However, I continues to receive a monthly bill and each month when I called, they had no record of my ever cancelling the service. I would then refax the original letter cancelling my service. (Deja vu? Groundhog Day? Same story every month.)
Finally, in November, I spoke to very first non-rude Customer Service person. She did cancel my service but only from Novemeber on.
So, the company states I owe over $300 for monthly service charges for phone service I cancelled in May. (I had actually stopped using the service in April!).
I have been reported to a collection agency as a result. In January, I faxed the original letter and another letter to the CS Manager (who I have never been allowed to speak with), along with a new letter letting them know I was going to file a complaint with the FCC if the matter was not cleared up.
The response? 1) Another bill and 2)a comment that unless I can produce a "receipt", they have no evidence of "ever" receiving a fax from me. (Funny thing is, the CS Rep. stated they had a record of receiving my fax in May and in Nov.)
This company is either grossly incompetent or grossly criminal in nature.