Clueless MTN

My MTN contract was changed to prepaid three weeks ago. Since then I cannot receive any calls. I can make calls but when someone tries to call me they get a 'this number does not exist' type of message. I have missed some really important calls due to this. I called the call centre numerous times and went into MTN in Middelburg Mall numerous times. No one can or is willing to help me. Everyone seems to be clueless on what to do. This is due to MTN's incompetence and they should fix it.