I'be spent 6 years so far explaining it on this very forum. Many times over. Maybe reading the posts, stickies, etc. would be advisable.
So to understand Vodacom's offerings and prevent shock bills, all Vodacom subscribers should sift through the mybroadband forum; your relevant posts; vodacomdata's relevant posts; the stickies; and educate themselves? That is what this boils down to. There is a common theme among the shock-bill victims - they simply didn't know. Now you can argue that ignorance is not an excuse, but that is only applicable if you (and the other networks, granted) do your best to ensure that your products are easy to understand; consumers are educated sufficiently; and you do your utmost to prevent these shock-bills. You simply don't...