No calls to landlines and no iMessage

I am a Smart M user. Since the beginning of this month I can't make calls to landlines nor use my iMessage option because i don't have actual airtime on my phone. I do have 144 minutes and 350 mb on my phone so this should not be an issue. I called 111 and I was informed that I have to load airtime in order for these functions to work. Why do I have to load airtime now, I've been using my phone without airtime since I had my phone and contract. This is not acceptable. How can Vodacom just change their terms without informing their clients. I am really unhappy about that.