Contracts don't hold any water

I would like to know why when we sign a contract for an agreed amount for 2 years is it possible for Vodacom to up the monthly installments? I am not a lawyer but surely this is *******? My monthly installment increased from R437.72 to R464.03 and when I inquired about it I was told that Vodacom had improved it's services hence the increase on the tariffs? Imagine how much money Vodacom are making if every contract was raised by R26.31? I find this totally unacceptable, why should the customers pay extra for Vodacom service improvements? In my view a contract is a contract and Vodacom are in breech of their contract with me.