Keeps on billing me for account fully paid

I have paid this account - ihowever I still receive letters and phone calls telling me that the account is outstanding. I have now received letters telling me that I have a mtn airtime account which is not paid. I am a cell c client. How do I make them understand that I dont use mtn?? I have all my receipts, but still, nothing is being done - I am at my wits end, everytime, someone says that they will get back to me -