Contract increase

How are Vodacom just increase the contract prices, when I signed the 24 month contracts is was for a certain amount of data, minutes and sms's at a certain price for a 24 month period. Now they just put of the monthly price, as far as I'm concerned this is breach of contract. I have tried to call their legal department, but conveniently that call never gets answered. I sure I will get a reference number and an automated reply here that means nothing.