Guam is currently using one area code. Guam does not utilize any area code overlays. Area code 671 covers all of the Guam.
Prior to 07/01/1997 Guam did not participate in the North American Number Plan and instead used 671 as a country code.
Prior to Guam participating in the North American Number Plan in 1997 callers from mainland United States had to make an international call to Guam. The FCC has established regulations which require long distance carrier to treat call to and from Guam as domestic calls. This also allowed calllers from Guam to use US toll free numbers. In Guam telephone service is regulated by the Guam Telephone Authority.
Changes and History of Guam Area Codes:
Area code 671 was put into service.