MCi Worldcom a real joke

I ordered a cellphone from Worldcom in June of 2000. My problem began when I was not getting bills fronm them for months at a time. At times upon receiving the bills, they were not itemized. I paid them anyway until I became frustrated and disenchanted with the service I was receiving. After being instructed that I would need to maintain my service for the year I decided to make limited calls with the service and bided my time until the contract was up. Still, I was receiving bills more than 3 months at a time. I made several attempts to call them even emailing them on 2 occasions with a request for an updated bill. Attempting to reach them by phone was a joke. Perhaps on 2 occasions I did speak with a human who could converse with me. At no time was I satisfied with any answers (or so called) given me. Not wanting a bad credit report, I finally decided to pay a last balance which they denoted was to be a final balance owed them in November of 2001. I thought this would have solved the problem. Today I received another letter from them indicating that they have not received my payment. I do have in my possession a receipt of payment. The payment was in the form of a money order which I obtained from the post office. I need help with this problem as they are threatening to file a report with the credit bureau. Any assistance you can provide would be greatly appreciated.
Thanks,

Robert
Philadelphia, Pennsyvania