I have a 36 month ce...

I have a 36 month cellphone contract with Vodacom and in the duration of not yet even two years my contract amount has increased twice and that's not the amount we have agreed on when we took out our initial contract, I went into the store and was told because of the annual increase on data bundles that's why our amounts has increased and they also increased our data and minutes which we did not request for but was told it can't be changed, I'm very unhappy because my understanding of a contract is what the amount is you agreed on and signed for should be for the duration of the 36 month of your contract