mtn overcharging me for my contract

I recently renewed/ungraded my contract with MTN last year November. I agreed to a 100 topup contract for R79 per month over 24 months. Since then my account has been debited with R114 that i did not authorise. I've made numerous calls to MTN (808) to try to get this rectified and have continuously been forwarded to various departments and still no help. I even emailed the complaints department a week ago and still no word. I want to know what the additional R35 is for? I did not request any additional services/products on the contract and did not give authority for the additional debit order. This has been a very frustrating and costly process and I'm still nowhere. The call center agents are never able to assist. I demand that MTN explain why I'm being charged more money on my contract and a refund of the additional (unauthorised) R35 that has been taken for my account every month. Its my money and they have no right. Or unltimately cancel the contract as there is clearly no consensus.