CONTRACT SIGNED A YEAR AGO INCREASE???

This is totally unacceptable! How is it that i sign a contract with Cell C, thinking that my contract will remain a fixed amount (if not what's the point of a contract?) for the next two years, i just opened my bill and it seems to have one up by R10, i have two contracts with Cell C, and has been a loyal customer for many years, never before has my contract just increased with no notification or consent. Again i ask WHAT IS THE POINT OF A CONTRACT ADVERTISED AS A FIXED AMOUNT FOR TWO YEARS IF YOU CAN CHANGE IT AS YOU PLEASE?? So next month i could just find that it was increased by R400 because of a line in your contract that states you can do whatever you want?? I'm sorry that ludicrous, why would anyone sign a contract if they knew that you can basically then bill them for anything you want, because it \was in your T & C's"(which no one points out when getting a contract) A serious breach of customer rights in my opinion"