Incorrect Debit on Account and refund not being actioned

I recieved a call from MTN to upgrade my contract, i agreed to the upgrade, my next debit order ended being half my annual contract rate. They incorrectly debited my account and even after logging two tickets with MTN, they have still failed to refund the incorrect debit amount. There seems to be very poor customer service for existing clients. They basically *****ing funds, because even if funds area credited, interest will still be lost.