Horrible service after being billed erroneously

I downgraded my service from an unlimited to a 30GB data plan due to bad network in my area. The downgrade was effected on the 1st of January this year. However, supersonic went on to debit my account for unlimited data. I requested for a refund on the 18th of January, this has been declined with the latest reason being that I have a supersonic device, which I never got. How difficult is it for the billing department to check in their own system if I ever received a device and secondly why do I need to call to be told that my claim has been declined. It is now more than 3 months, since supersonic debited my account erroneously and I have to pay with my time and airtime for them to fix their problem.