Unexpected charges on Cell C Fibre

Last month I realised that I was being charged an extra R15 on my Cell C Fibre account. I submitted a query to resolve this. I was called by Cell C and told that someone will sort it out. No one sorted it out. Next month my Cell C Fibre account is charged an extra R75!!! Again I submit a query to resolve it. Someone calls me back and tells me my query has been "escalated" and that those charges are not supposed to be reflecting and that someone will call me back. NO ONE HAS CALLED BACK AFTER A WEEK. WHAT IS HAPPENING??? I refuse to pay extra, my contract says I will pay R 899 a month. Nothing more and nothing less. Sort this out or I will cancel my debit order or switch providers!