Incorrect billing

MTN always reminds me why I should not use them. My contract ended earlier this year and I gave 1 month notice that my contract should not continue. I made final payment in April. Today I checked my credit history and it says I have a missed MTN payment, I called MTN to assist and I was told I was charged a cancellation fee. I am surprised how that is possible because the contract was not cancelled before the end date. No one is able to assist me, I am being transferred from contract to retentions and being told that they don't have access to the old system. I am so irritated right now as this is affecting my credit history. This is not my first complaint with MTN, even took it to ICASA last time, they always just mess up. Apparently now I owe them R1000 which I don't know where it is coming from.