Bad customer Service, Overcharged Bill

I was overcharged on my capped Cell C contract by more that R1000 I reported the mattter to the Cell C Fourways branch, that called customer care and sent me a reference number for my query and advised they would get back to in 48hrs. It has now been over a week, I have spoken to over 5 different people from cell c and still my matter has not been resolved. This is truly frustrating and unacceptable customer service. It's easy to take someone's money by mistake yet its not easy to rectify the mistake. This is not the 1st time this has happened to me, I truly don't know why I have renewed my contract with this company.