Contract Price Increases

With Vodacom's recent contract rate price hikes, why are they allowed to get away with this? The contracts are all sold as 24 months and give no clue as to the hidden fine print where provisions are made for this. When you chose a contract, nobody even hints at the fact that the monthly fees are not set in stone. When you open the marketing materials the contracts sate very simply X 24 months, yet this is not actually the case. I appreciate that there is some hidden clause allowing them to increase rates but is this really in the spirit of the law? Have they made it clear to subscribers that the rate is not actually over 24 months? Isn't this precisely what the consumer protection act in South Africa was designed to prevent? Surely there is a class action law suit waiting to happen here?