Vodacom want to increase contract monthly fees

i just got a message on my phone that says vodacom want to increase the monthly billing from R69 to R75 a month of which I signed a 24months contract. i went home, read the fine print of the contract that i signed and nowhere in the the contract could I find a clausse that says vodacom can increase the contract prices as they pleases. I went to vodacom shop in mosselbay mall where i took the contract out to file a complain and they told me they can do nothing about it, because vodacom head office said it is in the contract it is clausse 5.18, there is no clausse 5.18 in the contract. i just want to cancel the contract because i did not signed for a contract that vodacom said they can increase the prices is they pleased. There is no clausse in the contract that says they can do it.