Watch this video --- you won't regret viewing it.
This is valid, I have gotten some of these calls as have my friends, we never answer them but these criminals have gotten good at faking one of your “contacts” when they call, and if you answer, they could either try to sell you something, or worse, ask for personal information since they need it for an emergency, or to qualify you for a free cruise or something. If I had not seen this on my own phone, I would not think it is a risk, but, I feel sure this is a risk.
App – Robo Killer seems to be effective in stopping spam calls to smart phones
yet another App for blocking unwanted calls
Mr. Number seems to be a well rated app to stop calls as well