Can Vodacom just increase prices????

I received emails from Vodacom yesterday informing me about the increases in the price plans... <br> <br> As far as I know, my contracts is fixed for 24months as signed by me<br> <br> Are they allowed to just increase our price plans and only notify us via email? Isn't a contract price fixed for the period of the contract?<br> <br> This is like renting a house on contract for 12 months, and after 4 months the landlord just decides to increase the rent???<br> <br> Vodacom is really treating customers unfairly now!!!!!