This time I was able to respond to a live call. Turned out to be Heartland Hyundai calling to receive customer feedback on car service they had undertaken a few days ago.
So, it was a legit call, though it may bell have been scheduled by a computer.

Number calls everyday, never leaves a message. Call back and it's just someone named 'Sarah' (no last name or company) on the voicemail.

January 2016 missed call on two successive days. Calling back got no answer.
Will update if anything new comes up.