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).