Account incorrectly billed a sum of R1 800

My account incorrectly billed a sum of R1 800.00. I requested a refund about 3 weeks ago and every time I call to follow up I'm being told the refund has been approved and that it will take a couple of days to reflect in my bank account. I called again today and the lady I spoke to told me that the amount was never actually refunded to my bank account and that it was credited against my account with mtn. I'm very unimpressed with this especially since I might have to pay interest for being in overdraft as a result of the incident.