Cell C really *****s from the people!!!!

I went to the Carnival Mall Branch in May 2014 to get the settlement amount for my contract and they gave it to me and went to the bank and paid the money and I e-mailed the proof of payment and 13 months later there is surprise debit on my account from Cell C and apparently I should have called them to say I'm cancelling my contracts and the amount I paid was recognised as a advance payment...how stupid can I be to take a contract then suddenly make a an advance payment???also the big question is why would I pay my settlement amount for the contract if I'm not settling/cancelling my contract?? I cannot be another Cell C victim i'm fed up....