Vodacom deducting more money than what my contract says

I amended my contract with renewal last year with Vodacom. I then notice that Vodacom was taking around R400 more than what my contract says. I then informed Vodacom regarding this problem. After the 3rd incorrect billing , they manage to correct the billing. I then requested a refund or credit. After plenty of phone calls, emails.. driving up and down, Vodacom just simply said : “Sorry, they will not be refunding me or give me any credit” after admitting on paper that more money was indeed deducted. Thank you Vodacom Nelspruit for the day light ******* !