more than likely a debt collector fishing for information on you,,

Why? Because they can, as there are no teeth in the do-not-call laws. And there is much $ to be made thru telemarketers because fools keep sending $ to them through purchases or outright fraud, which continues to keep them in business. Further, not even sending $ benefits the scammers as they still sell your verified active number every time you answer their unresponsive calls. They then sell the numbers as a list that companies who desperately want to separate you from your money are willing to pay money for. My wish is that home intrusion via the telephone and emails would be taken as seriously by law enforcement and the justice system as physical home intrusion. i see no difference; it is just as disruptive to most of us and in some cases can actually cause medical/bodily harm to disturb the homes with members who are critically ill, on hospice, etc.
But the greedy simply do not care whom they hurt.

Called twice in the last three days.  No msg left.  Caller ID says Resrch Study.

this number called offered vacation for doing short guestions about government issues, questions are interesting but...if you answer you want short vacation..3 day 2 nights in resort city of your choice, they say you will get called back within hour, then number that calls back is  971-220-1000 and says "vacation reward "for caller id, this company is one of those time share advertising companies that will let you go if you go to timeshare sales meeting, and financally qualify

I pick up the receiver and say nothing, mainly to keep scammers from leaving a message. Your voice triggers the speaker to either leave a recording or a voice mail. It's interesting , the voice on the other end usually just hangs up. I found that the calls have almost stopped. I was getting 8 or ten calls an evening,  some evenings We get no calls. I carry my phone in my pocket when I'm home and catch it on the first ring. jan