Received call from this number as unavailable left no message. I figure if they don't want me to know who is calling they don't need to know if I am home so I don't answer.
If you call the number back and get a NON-WORKING NUMBER message it's pretty certain the call originated from overseas (India, Mozambique, Indonesia, etc). They, somehow, can put in a valid looking number to show up on your caller ID, in hopes you'll answer.
If you call the number back and get a sales pitch it could be from a call center anywhere working thru a US phone service.
I can't tell that reporting these numbers does anything but encourage them. I get so many of these on a daily basis I couldn't afford to have Verizon or ATTput blocks on all of them, at $5.00 per number; even at $2.00 I'd be well over $100.00/month. Some DROID smartphones have a feature to direct a specific number directly to voice-mail. Since the callers never leave a message you're never bothered. Otherwise, the best I've come up with is to put the number in your number log as DNA (do not answer), give it the quietest least offensive ring in your ringtone library. (I've been told some phones have a silent ring tone. I hope this will help some of you.