Negative listing for something I have paid

I called Cell C to cancel my contract and I requested the cancellation fee which I have paid in full and my contract was then cancelled. After a month or 2, I was called by Cell C stating that I owe them about R1095.00 which I don't understand where was that coming from as they are the ones who gave me the cancellation fee and I paid it. Only to find out that I have negative listing on my credit profile by them for something I don't know. Can they please remove this.