No one is taking responsibility and Im getting billed for it!

I was offered an early upgrade on my MTN account for a data only package at R89.00 pm which i happily took up. there were no additional costs or levies as I'm sure you can happily pick up from the call. I was subsequently billed R580.00 instead. i immediately called your call center and a claim was lodged for the billing to be corrected and a refund back to me. today i called to follow up on my refund only to be told that i am only getting refunded R200 odd rand as aposed to the R491.00 and that i have been billed incorrectly again this month. no one is taking responsibility for this and I am getting charged way above the agreed rate!