Double debit

I have a cellphone contract with Edgar's that is administered by the above. I was double debited on the 25th of June and got an apology sms on the 28th promising to refund me within 5-7 working days, and today is the 13th working day. I've been calling them and only to be sent from pillar to post and they always promise to call me back within the end of the day which they don't. Everything I call am being told that they still waiting for the finance department. My worry here is that come the 25th I will be debited again without getting my refund. I find this as a bad service how can I wait for a refund for almost a full month.