Violating signed contract

I have been a loyal customer for 4 years with Cell C. After careful planning I chose a Contract that caterers for my needs and also fits my budget. This month I realised my billing has increased. In the contract i have signed it stated my billing for 24 months.<br> Who gave Cell C the right to violate a signed agreement? <br> <br> I went to into Cell C's offices and they told me the notice of increase was given in the billing posted to me. The last posted billing I received was in October 2014 and still that notice still doesn't apply to me as I have a signed agreement on a specific amount.