Vodacom implements increases during contract period

I have a 2 year contract with Vodacom and received an sms yesterday that they were unilaterally increasing the premium during the 2 year period, even though we had agreed terms for 2 years. I understand they have done this to all customers. They reported a profit after tax of over R17 billion last year and still they want more! This is an absolute abuse - to agree terms and hook you in for a two year period and then change the terms during that period. They say their contracts allows them to vary the terms at their discretion. What contract allows one party to change the terms during the contract period? Unheard of and *******. I am considering instituting a class action against this ******* and immoral action by Vodacom. Greedy!!