Dunno what this is, but they've now called 3 times: twice in less than 5 min Thursday around dinnertime and Friday in the morning. No VM left by any of the calls.
White Pages says the carrier for this number is: "Twilio VoIP in Cambridge, MA". Wikipedia says: "Twilio is a cloud communications company based in San Francisco, California. Twilio allows software developers to programmatically make and receive phone calls and send and receive text messages using its web service APIs."
Gotta love it! Used to be the scammers needed some relatively-expensive special-purpose hardware to make these nuisance calls. Now, through the miracle of the 'net, there's NO cost barrier to entry, and EVERY Tom, Dick and Harriet can get in on the action. Real Progress.
Entered number into my phone's Do Not Ring list. Problem solved, for me at least. ;)