R10K taken from my Bank account after we already paid up our contract

In October I closed my contract as I was immigrating and paid my contract in full - over R12K. At the end of November MTN took another R10,889 from my account without notice after the number / contract have been closed already. I'm now sitting overseas and have been calling every week or two weeks to only be told yes we know it was a fault, we've spoken to our manager and the money will be credited to you within the next week. This is now 2 months later - still no money. I don't know what to do anymore. R10 k is a lot of money and my account is now overdrawn because of this and I am payin interest and not getting this resolved. How difficult can it be just to give me my money back that should not have been taken from my account in the fist place!!!!!!!!!!!!