MTN Falsely listed me after paying early for my contracts.. disgusted

I have had a mtn contract that was paid in advance at the store. I have receipts to prove that I paid. And these people go and list me for not paying for two months. If I didn't pay services would have been suspended. I have done the right thing as customer and paid what was owing. How do I get them to remove it. I have been sending emails every day calling and still these people refuse to do anything about it. Its so wrong cause I always pay all of my accounts on time now to have them do this is another story. Its affecting my credit report which I have maintained to a score of over 600 and they go and do this.