Negative listing from MTN

I defaulted on my monthly installments for 2 months in which I repaid and got my account current and active. I now have a negative listing on from MTN though I do not have any arrears with them. I have been trying to get them to remove that listing for months now and they keep giving me reference numbers but no one seems to be able to assist. I am unable to open any accounts now due to the Negative listing from MTN.