Cannot get MTN to change my contract to prepaid

For about 3 months I have been trying to convert my fully paid contract to prepaid. The first time I called after holding on for 20 minutes, I was told that I should phone back once I have used my minutes as I would lose it once I convert to prepaid. I called about a month later and was told that I cannot speak to the department directly and the call centre agent will submit the request and it will take two days for the conversion. My reference number is ***. That was about a month ago and I am still waiting to go onto prepaid and in the meantime MTN is debiting my account.