It depends on how you responded to a previous email. However, they may have your personal information from some other scammer. The main thing is, delete the emails and don't send them any money.
e.g. http://travel.state.gov/visa/"Some websites and emails try to mislead customers and members of the public into thinking they are official U.S. government websites. These websites are designed to appear official, and often have images of the US flag, US Capitol, White House, or Statue of Liberty. What these websites and emails are missing is the ".gov" suffix on their addresses."
Maybe you sent them some personal information or they found your phone number by searching on the net. In any case, probably the best thing you can do is to see if the phone company can block calls from them.How they got my phone number?? I just want to complain about this.
Name : Jack Lewis
Address: 73 Queens Avenue
London, N20 0JB
The payment must be sent to the above U.S. embassy agent address in United Kingdom because the U.S. Department of State decided this based on the diplomatic relations with your country.
After you send the payment follow the next steps:
Fax the Western Union receipt at +44 207 691 7968 !