After reading past comments, I would say this is an unsafe caller, probably phishing for sensitive info to use for a scam.
I routinely do not answer calls that are not in my contacts, so when a caller will not leave a voice message that identifies the caller, and the reason they called me (which this caller did not do), I always block the caller.
I mess with them also. I usually try to keep them on the line as long as possible. Hope they get in trouble by staying on the phone with me for half an hour or so then, to have me call them out without getting a dime. Figure it keeps someone else from possibly falling for it. I have gotten some of them so mad they resort to saying bad things about my mother. When I tell them it's true they get even more mad and hang up. Now my phone terlls me it's a possible scam. So I was able to tell them that caller Id's are getting smarter and calling them out. Maybe if enough of them hear that, they'll stop.