Billing after contract has been cancelled.

I have had a contract with MTN for more than ten years, but after moving to Switzerland a year ago I obviously needed to cancel it after the contract ran out. It didn’t make sense to keep the contact going as they suddenly adjusted my credit limit (have never missed a payment in 10 years) and the phone kept cutting off when I was overseas. Also I have discovered that by international standards their rates are a complete *******. The people at their Rivonia branch told me that all I had to do was call ********** and ask for it to be migrated to prepaid and I could keep the number. I duly did this two of three days after the contract ended and was promised that all was in order. Problem is, a month later I was still getting bills so I called them again and this time they did the cancellation. So now I have paid an extra two months for a service which I did not receive and do to an administrative error on their part. Another 25 minutes on the phone (from Switzerland) and eventually I am told by the manager (Marvin) that he cannot help me because the “process” does not allow for it and I need to speak to another department- he promises to transfer me to their supervisor. Needless to say the call goes nowhere and after ringing for another five minutes I give up. If they didn’t have a monopoly they would be out of business. Unbelievable! My latest reference number was ********** 89