Cell C changed my contract.

I have a data contract with CELL C. This contract was on a month to month Basis and Cell C decided to change this to a 24 month contract without calling to inform or emailing to confirm if this change will be excepted. Now I want to give 30 days cancellation notice and I cannot cancel because they changed the original agreement. When you sign up they do it on a recorded line but they can change the whole contract without so much as a courtesy call.