Price increase on a Top Up contract

I'm extremely peeved at this company and it's pathetic customer service consultants. I took up a \straight up 50 top up"contract with the company where the initial agreement was R129 monthly for the 24 months. Suddenly it increased recently to R135 without any communication to me. I called and enquired about this amount and they told me all sorts of tedius nonsensical stories that it is stipulated on my contract that prices may increase at any time. I have the document I signed when taking this contract and there is nowhere in black and white where it's written. Why am I being ********** and told bogus unsubstantiated trash when I've been nothing but a loyal customer to Cell C. Or is this how they treat customers now? I want this contract cancelled if they are going to continue treating me like an idiot. """