Failed to cancel contract

I had a Cell-C contract around 2013 which I closed around the 18 March 2015. Years later I discover that I owe Cell-C over R3,500 which now reflects badly in my credit report. I have paid up my account with Cell-C and not a single day did I receive communication from them to say that I owed any amount. Out of the blue I owe them R3500 for the account. How do I suddenly owe over R3500 for an account 1) I have closed 2) that I have not used 3) that was settled? Had there been any amount owed why was this not communicated to me on time? I had to find out on my credit report...