Contract account increase without concent

I'm a contract client with 4 nrs on my contract.<br> I received a sms from Vodacom stating that there would be a price increase from 1 May.<br> This is not acceptable as I signed a contract for 2 years, not something that can just be changed one sided by them, just as I cant change the contract details from my side.<br> <br> And sending this by sms is probably the lowest as it should have been done via a letter.