Caller ID says "3397077549" (The default format is (xxx) xxx xxxx; so they spoofed the caller ID to look vanilla but have failed).  They leave no message.

I call back after business hours (using *67 to block my number).  Their outgoing message is, "Hello, you've reached MDS communications.  We're a telephone fundraising company that specializes in calling for nationally recognized charities, non-profit organizations, and political committees.  Please, accept our apology if you do not wish to be called particularly if you have signed up for the national do-not-call list.  As you already know fundraising calls are exempt from national do-not-call lists, and many people who have signed up in order to block commercial calls still like getting calls from their favorite charities.  If you'd like to receive a call back from us, please, press 1.  If you'd like to be added to our do-not-call list, please, press 2."  ( I never do the do-not-call list as it can actually make things worse).

Likely they are what they say they are, someone calling for a charity or a political campaign.

This number called yesterday, left no message. Called again this morning, no message. ID says Arlington, MA.
One more call and onto the Block list.

Dec. 22
did not recognize the number-did not answer

I called the # back it is a "charity" line so it is exempt from the do not call list.  If you listen to the entire message there is a way to opt of of them call, hope it works.