More than likely a "robo call" from the DNC that's programmer not to leave a message but to only interact with a live person. I get these often as a Democrat and donor.

I get at least two or three calls a day from this number; always shows as "UNKNOWN NAME."  This has been going on for weeks and weeks.  Call is terminated after my voice-mail comes on. If I pick up, there is never anyone on the line.  At least these calls only come in during the daytime (Pacific time zone).

Got a call from this number just now and answered it. A woman called saying that she was calling on behalf of the Democratic National Committee.

These scum bags call from many different numbers.  I block them every time.

This number was from the Democratic Party. No one answered when I picked up the call. I called them and had my number removed from the list.