> Received the call - did not answer - have just blocked the entire 571 area code.
Your call MAY have been a ROBO Call, but some of the 571 area code calls are valid if you interact with the U.S. Government. For example, 571-272-xxxx are telephone numbers for the U.S. Patent and Trademark Office, and 571-273-xxxx are fax numbers for the U.S. Patent and Trademark Office. See:
http://www.uspto.gov/about/contacts/phone_directory/index.jsp