Price increase on contract

I have a 24 month contract with MTN which I signed in November 2013 of R319.00 per month. In June 2014 I received a bill of R329.00 per month. Various calls to mtn holding on for more than half an hour I finally get hold of mtn (21/07/15) and the lady at the call center says there was a price increase, I don't understand how can there be a price increase when mtn and I signed and obliged to the contract of R319.00, without informing me mtn just deducts figures that they want in this days economy one can not be unwise and the fact that I held up my end of the contract and is paying the required amount per month why cant mtn do the same. From what I understand a contract is a binding documents and one can not amend it as and when they feel like doing so.