Vodacom breached our contract

I took out a contract with Vodacom and signed for a specific contract amount for the next 2 years. Recently I received an sms of a price increase. When I phoned Vodacom since I was not happy because I signed a contract for 2 years for a specific amount and when I checked the forms I signed, there is nothing and no fine script saying that there is an allowance for a price increase, they told me that I must check section this and this but there was no such section on any of the forms I signed and when I kept persisting that they must show me copies of the forms where I signed to consent to the fact that I am aware of any price increase that might happen in future, they then put me through to the legal department who then said that there is no such section, meaning that the previous person **** to me, probably in the hope that I wont take the matter further if I thought I mistakenly signed where there is such a clause (which does not exist). if they can just increase the amount any time (which they said was their right to do) then I would never have signed a contract because what if I cant afford the increased amount (which can amount to anything over the next 2 years)