It's robocalling and quite widespread and getting worse. If your carrier supports it, sign up for NOMOROBO.
I can't tell if your carrier supports it so check there and at https://www.nomorobo.com/
CNET reviews it at http://www.cnet.com/videos/how-to-stop-robocalls/
I never answer calls on my iPhone when I don't recognize the number and I get many of those. In the past couple of weeks I've been receiving at least two or three a day listed as "unknown". They don't leave a message. I won't answer but am curious what this means. It's annoying!