Cancellation of a contract

I had a 2yrs contract with mtn which ended last month,was paying R199,I've called mtn in September to cancel the contract and they told me to call or send email 25 working days towards the end of the contract,which I did so,to my surprise they've tried to debit after the cancellation of the contract and now I got a statement that I'm owing them R778,so I want to know that R778 is for what because I've canceled the contract.