Caller ID says "Cambridge  MA"  (although another site claims the number is in Everett, MA).  They leave no message.

I call back after hours (using *67 to block my number).  Their message identifies them as the Donor Privacy Protection Line.  They are basically a hired-gun telemarketing firm.  They are calling on behalf of some charity (they represent many).  There is an option to leave my number and the charity that's calling to be removed from their calling list.  Since I don't know which charity called, I don't even know how I could do this.

They have called before using other numbers from the 617 area code.