Cell C contract

Has contract with CellC for years, paid all well. Cancelled my contract in time to my surprise I received an sms saying that I failed to pay and I owe them and my account has been handed over. Tried solving this telephonically as going to the shop they will say I must phone but I could not be helped because of security questions not matching. Mind you I have had this contract for years, I have changed addresses, lost phone, got married but I had to remember information of years back. Highly disappointed and want my name cleared.