increased subscription

I upgraded my cell phone in February 2014 and my subscripption was R149 for the next 2 years. In February this year Cell C changed it to R159 without notifying me. When I queried this morning the lady (Nkosinami) said an sms was sent to but I never received it. I know its R10 but I am a pensioner and I have to watch every cent I spend. My contract is for 2 years. My understanding of a contract is that there should be no changes during the period of the contract without acceptance on both sides. I was given 2 other numbers *** or ***) for someone to help me. Yes both numbers went to voicemail. I have been with Cell C almost from their inception but now I'm not so sure about them. Will they increase their subscriptions whenever they feel like it. Maybe its time to move to another service provider. Or maybe Cell C won't care