...Free is something that you get for doing nothing. I'm sitting on my couch watching TV and I receive an SMS that I got some reward from a giveaway or something from somewhere I might have shopped or done something that wasn't specifically for that purpose, now that can be called getting something for free.
But if you do receive such an SMS that indicates that you have received some kind of reward, then you should not rejoice prematurely, since this is most likely a scam. This is exactly how you should treat everything free, not forgetting that "the only free cheese is in the mousetrap".