Got a call from this number on 3/15/2017 at around 2 PM. Caller ID showed "Out of area 1-720-666-6531." Let it roll thru to voice mail. No message left. Blocked the number.
UPDATE to my previous post:
In the week since my original posting above, I've received several more hang-up calls from two similar numbers: 1-720-666-6524 and 1-720-666-6527. So instead of just adding more individual entries to my Block List, I went ahead today (3/28/2017) and blocked the entire area code & prefix combination. That is, I've blocked every incoming call that starts with 1-720-666-xxxx, regardless of what those last 4 digits are. So whoever this is who's calling, they'll have to start spoofing a different prefix now -- at which point I'll just block their new prefix as well.