I have no clue who this is. The message left was just a grunt.

Didn't answer and they didn't leave a message.

They called back. No message left this time.

Got a call from this number yesterday (5/31/2017) at around 4 PM.  Caller ID showed up as "Out of area 1-720-306-0851".  Let it roll thru to voice mail.  Whoever it was they didn't leave any message.  If you call me and hang up without leaving a message, you get blocked.  So ... I added this latest number to my phone's Call Block List.  One strike, and you're out!

no message ..no balls