I think you have to sign up on the "Do Not Call" registry every few years. It also only stops calls from "unknown numbers" IF, and I stress that word, IF the people calling even get the list and go through it to see who doesn't want to be called.
Got this same call. Answered and it sounded like the national emergency broadcast signal. Over the line calling the President a traitor and should be prosecuted for using the emergency broadcast sound in such a manner to misrepresent the call.
You can go one step further and report this to the FCC here:
http://www.fcc.gov/complaints
or simple google how to complain with the FCC and it will direct you if you are more comfortable finding the site on your own as I did.
We gotta stop these nutjobs. Take 5 minutes to do more than complain on sites like this (while totally cathartic, I'm guessing that is all they do for us) and let our government know when you have had enough!