Existing contract client price increase? WOW

We received an e-mail notice today of package subscription increase. Is this legal? We signed a upgrade contract with Vodacom for a period of 24 months. Fixed benefits and fixed subscription to fit into our budget. I am not allowed to change the terms and conditions as I like. How is it possible for Vodacom to do that? Because they are big and mighty? Can't find any such mentioning on my signed contract that gives Vodacom permission to charge any higher subscription fee they seem fit. The telephone number given on the \report"section"