Cancellation of contract

I have a contract with Mtn, August was suppose to be my last month with them, as my contract has come to an end. I don't want to renew my contract with the service provider, i want to cancel. My Installment is R398 but for the past 3 months it has gone up to R410, without my knowledge, no one told me what the extra amount is for. I now want to cancel my contract and August was suppose to be R398 but when i called the call centre today they told me they will deduct R756.40 and for what they did not say. I received my statement this morning and it says R0.00 Current Outstanding but the Invoice says R403. . No one can explain to me all these amounts. I am really disappointed with MTn. I have been their customer for a long time and now that I want to cancel, there is additional costs. I am really disappointed.