Thumb Sucked Billing System

I am trying to find a solution on how to deal with account queries with MTN. This was not my first enquiry and worries me to think of how the systems operates at MTN. Is it a thumb suck process to ascertain what a monthly account is, and we just debit what ever we feel like debiting? Let me further explain, I have 2 contracts with MTN (Unfortunately) neither account exceeds their allocated data or airtime and we always have a carry over yet every month the debit flatulates. Last month was R589.25 and this month was R848.01. Every time I phone I am told I am on an uncapped line and every time I am told this, I request the lines to be capped. Regardless of lines being capped or not, we do not exceed our bundles therefore there should be a standard charge every month. I request that my enquires are escalated as the call centre is not equipped to deal with such enquires. I also request that an audit of my billing and telephone calls to MTN over the past year be done and a detail report of how you got to your fluctuating figures reflect. Please revert back with the credentials of the Manager whom will be dealing with my enquiry and a expected date of when I can receive a detailed report.