Incorrectly billed! No help from Cell C

I've upgraded start of May. By the 20th May I never received my device. I phoned in and canceled my upgrade. It was done. Lady at retentions said I could stay on for R30 pm so they won't loose me as a customer. I agreed. Now I get billed R801 instead of my normal R 320. I've phoned in numerous times. I very got a case and ref nr. Nothing happens. Every time I call in they say it will be escalated. Nothing happens. It's like speaking to a wall. I'm frustrated and don't know what to do anymore.