Sudden increase of line rental fee

Vodacom has increased my monthly line rental in the middle of my contract. I called to query it and the customer care person says that it is in my contract that they can change the value at any time. (probably true, I don't recall this being pointed out at the time). I signed a contract for a particular value, so how is it legal to increase in the middle of the contract! What if someone can no longer afford the increased rates, will they now get blacklisted for being unable to pay a contract fee they didn't agree to. I moved to Vodacom expecting better service. I was very wrong!