Over charged for no reason on my cellphone contract.

I noticed a billing adjustment on my bill last month. I was charged about R26 extra for nothing. Upon calling the call centre twice, no one knows anything and the issue keeps getting escalated and when I follow up with the ticket number, the case is still open. In the meanwhile, mtn will debit extra every month. I was not informed of any increases to my bill, neither did I make any changes or did I take any additional products from MTN. When you take a contract with MTN, it states a fixed price for 24 months. If this issue is not resolved, I am going to cancel this contract when it ends and port my number to another provider.