Typically, these calls come in when I'm not home. Voice mail picks up and, of course, no message is left. I only know who's calling by checking out 800notes or whocallsme.
When I am home and get a call from an unknown number, I just "answer" and immediately hang up. For the few opportunities I've had to do this, they've never called back. Of course, I've got no idea whether other numbers arose from the ashes of the phoenix and got my number.
Their latest tactic: spoofing your own area code and telephone exchange and using four random digit numbers at the end of it. That way, you THINK that it's someone either in your organization (or city) and are tricked into answering the phone. If it's not in my contact list, I now don't answer the call. If it's legit, they can leave a voice mail. These scum never do.
Asked about me taking advantage of a free home security offer. Hung up right away. I find that answering then hanging up works better because they won't call you back instead of just ignoring the call.