Complaints Department at MTN doesn’t work

I have been requesting a refund of approximately R12,500.00 from MTN, where they ********ly went into my bank account and deducted this amount for a contract that was settled a year earlier. I have escalated the issue all the way to their complaint department and they have ignored my request. The credit is reflected on my MTN account but they refuse to refund me. Can someone please advise on what to do next!!! Highly disappointed!!!