I am blacklisted by Cell C

Handed over for contract that I have canceled I wrote to hello Peter in April about my contract. I canceled the gave an assurance that my contract is canceled my number was converted to prepaid but now I went to the bank I was told that Cell C has blacklisted me. Now I am receiving calls from company called css requesting payment for the money I don't know, please let not play games who ever is responsible for this must sorting out before I take this matter to court.