Overcharged and disappointed.

I have been overcharged on my monthly debit order by over R2000, and no-one at Cell C is willing to assist in rectifying the problem. The call center agents can see there is an issue, agrees its a mistake but is powerless to fix it. You are unable to ever speak to a manager as their calls are screened, and they are always "on lunch" or "Not available. Why then have a manager if they cant assist a client. I can't cancel the debit order as then my phone will be switched off. So powerless to fix the issue. After 15 years with Cell C I think i need a new network provider.