Won't refund my over payment after contract finished

I am writing on behalf of my daughter. She had an MTN account. The account was about 5 months old, when she decided to pay for it in full, so that she didn't have to have any debt. She called the centre, they gave her a figure, of which she then paid. They then contacted her telling her she had overpaid and there was a +/- R530 credit and they will refund her. We submitted the claim in early July. Up until now... nothing has been paid back. We've sent out numerous emails, called three times and every time we are told - it's being processed. What am I supposed to do, to escalate my daughters refund?