Based on the comments on other websites, this appears to be an "active number" harvesting service. They call just to see if anyone answers. If you do, your number goes onto a list they sell to telemarketers. They never have asny intention of talking or leaving a message, and in all likelihood it is completely computer controlled. You could not talk to them if you wanted to, because it isn't manned by anyone. Just a robo-dialing computer calling all numbers in sequence in a given area code. That's why it doesn't matter if you have an unlisted number.
 Apr 29th, 2016
I have an unlisted number.
 Apr 17th, 2016