http://smallbusiness.chron.com/fax-vietnam-30069.html

To send a fax from Vietnam to the United States, you need to use the international code for dialing the latter country -- which is 00 (two zeroes) -- in addition to the 10-digit U.S. telephone number (3-digit city code + 7-digit number), since the fax is being sent through a telephone line. It may take the fax several seconds longer than usual to connect to the U.S. number since the call is being placed overseas. Because of the distance involved, it might take several attempts at sending the fax before it goes through.

E.g.: Fax to (718) 425-0543, press : 0017184250543