MTN will not refund my money

I had a data contract with MTN which I cancelled well in advance of the Contract Expiry. MTN confirmed the cancellation at the time. I then accepted that when the time came around MTN would stop deducting Unfortunately I neglected to check the expiry date and MTN kept on deducting. I then contacted them and they eventually did cancel the contract and confirmed that the account is now in credit.. On 2 October MTN confirmed to me that they will refund the credit into my bank account. According to MTN when I called today they refunded the money on 7 October but there has been NO deposit from MTN into my account. I have called 6 times and each time it is the same story. Each time I spend about 20 minutes on the phone. They do not refund the money and nobody comes back to me to follow-up. All I want is for MTN to deposit what they owe me into my bank account.