Price increase

I received an sms to inform me that my top Up 135 is now a Top Up 140. I signed a contract for two years. What is the reason for a contract if they can increase the contract price at any given time, with any given amount? <br> <br> They are forcing all their subscribers to this price increase. SHOCKING! Shame on Vodacom! I hope they loose a lot of customers! My contract expires in August this year. Time to start shopping around for a better deal elsewhere