Vodacom increases pricing on contracts in use

I currently have a contract with Vodacom that I choose over a year ago under certain prices. Now I get an email that they are increasing my prices even though it is still several months before my contract expires. Surely this is ******* as I never agreed to accept a contract with fluctuating prices. Everyone knows that when you sign a contract you will be paying the fees that you agreed to when you signed and not have to worry about increases until you take on a new contract after 2 years. Although it is only a few rand now, if we dont stand up to this abuse by big organizations then this could be the start to many *******s to come in the future.