Increase of Contract without notification

Checked my Vodacom statement just to find out that I have been billed more than what I signed up for when I took out the contract. When I called the customer service, they advised me that I have signed a clause allowing Vodacom to increase my premium at any time. My concern is that Vodacom is covered and can increase their premium at any time but when you as the customer wants out of your contract, you must buy yourself out. How are we as the customer covered in that way? Also the consultant does not advise you of this when you sign up. When you sign up a contract, you take something that is going to fit your budget. At the end of the day you can end up paying double the amount that you signed up for as Vodacom has the right to increase whenever they want to.