Vodacom unilatirally breaches contract

I received an sms from Vodacom saying they were increasing the prices of my data contract by over 10% a mere six months into my contract. I chose the contract because of the cheap price. I would NOT have taken out the contract at the price they are increasing it to. I specifically asked the consultant whether the price would remain the same throughout, and she confirmed that it would. I have since done some homework. Vodacom is allowed to increase prices, but then I am also allowed to cancel the contract without penalties. However, after having contacted both a branch and the call centre, Vodacom refuses to cancel the contract without penalties. In fact, they are charging penalties equal to 12 months' fees of the remaining 18 months. I find this unacceptable. Either Vodacom must honour the contract we agreed to at the price agreed, or I must be allowed to cancel the contract.