Robo-call? Caller would not respond when answered, but would call back immediately as soon as would hang up. I suspect it is one of the Obamacare telemarketers who have essentially stolen people's numbers.

Robo Call, Asked If I preferred Trump or Clinton (you know the routine).  Answered that I preferred Trump over Clinton and then cut off.  Did not ask gender, age, party affiliation.

Called twice after 7pm within 20 minutes.  I don't answer numbers I don't recognize. When the machine picked up, they didn't leave a message.  C.I.D. said Milwaukee, WI but any caller I.D. number can be spoofed.  We can't trust the C.I.D. any longer.
No idea who is calling but I must assume they don't have any legitimate business with me or they'd leave a message.

Just called right now on cell, no message. Going to block them.

Called three time over two days; no message left.