Took extra money from my account - unable to explain why

At the beginning of November, an amount of R2700-odd was taken from my account. My bill was only around R1500. I immediately went into my nearest MTN store to query, but all they could do was call the call centre themselves. Everyone agreed that there was no reason that they could see for the money to have been taken and said they would escalate it. I heard nothing. I called the call centre myself. Started the whole process over again. Received a "resolved" sms, with no explanation and no refund. Called the call centre again. I have now called the call centre FIVE times, and relogged my complaint. Each time, I either hear nothing, or receive a "resolved" SMS with no explanation and no refund, and the only way to find anything out is to call the call centre again. I have tried on twitter, facebook, the store and the call centre. It is starting to seem that the only option available to me is to visit their head office at great time and expense to me. It is unacceptable that money has been taken from me with no explanation and no follow-up. I doubt I'll get any satisfaction here, but I thought I would try since they surely aren't going to refund me my hourly rate and petrol to drive out to their offices in the middle of nowhere BECAUSE NO ONE IN THEIR STORES HAS ANY COMPETENCE TO RESOLVE ISSUES.