prepaid number billed as contract

I have been calling MTN customer care dept and have sent numerous emails regarding an incorrect billing on my account. I have been billed for a number which is on prepaid. My line has now been suspended which clearly indicates that my query has not been looked into. I have never received such bad service and I'm worried as to how this will be affecting my credit report and weather once its fixed can they go back and fix the credit report as well.