MTN *****ing my money

I upgrade my contract 3 months ago. My new monthly amount to be paid must be R429. For the past 3 months I pay R528. When I query it, MTN said they made a mistake by adding additional VAT and that they will reimburse me and rectify there mistake. I still pay R528 without reimbur*****t, now MTN says "SORRY IT IS LIKE THAT AND I HAVE TO PAY THE R528 PER MONTH. In my contract it says" MONTHLY SUBSCRIPTION FEE INCLUDING VAT R429.