I received a call from 713-000-0000 today. I did not answer.
The best advice that can be given is to NEVER pick up your phone if you are not positive who is calling. Let it go to voice mail. I figure it is better to let it ring 5 x before going to VM so it takes up their time from doing it to someone else.
Would the NSA PLEASE-DO SOMETHING about this peculiar form of domestic terrorism?