Failure to refund fees.

I have 2 accounts with MTN South Africa. I called them to change my banking details for the debit order to go through my new bank account. they still debited my old bank account which did not have enough funds in it. The bank charged me a fee for insufficient funds which they said is the fault of MTN. I called MTN 3 times to refund my fees amount. They keep on sending me messages that the query has been closed without refunding my money. Please help me resolve this issue. Thanks.