Overcharging

I have a very serious problem with Vodacom recently. I upgraded my contract with them from May, since May Vodacom has overcharging me on my debit orders with no quality reason,they lame excuse every month is that I've taken an airtime advance of which I didn't even take they even fail to give me sufficient proof of that . So I'm tired of Vodacom and their employees because every time I call my matter is not resolved and they always tell me that the manager will get back to me and nothing happens until I call . So im actually fed up and I want my money which they have been debiting extra from my account. I'm totally fed up and frustrated with their services. Every time when I want to speak to the manager I'm always given excuses as if I'm a small child who doesn't know what he is talking about.