Area code 684 is the number assigned to telephones in American Samoa. The area code system is part of the North American Numbering Plan, NANP.
Access to telephones in this area from telephone outside the region need to be aware of the country code, which is "1". Business cards and letterhead may list their telephone number as +1 (684) xxx-xxxx, where x is also a number. 1 is not only the long distance access prefix within the United States but is also the country code.
Area code 684 Wikipedia
As this jurisdiction is part of the North American Numbering Plan, callers from outside the NANP area dial into this area as they would dial any number in the US or Canada (or any other NANP member), by dialing their international access code, the country code 1, the area code 684, then the phone number.
Jurisdictions within the NANP area dial as they would any other long-distance phone number, as 1-684-xxx-xxxx.
Although cell phones can be dialed as described above, the cell phone companies have not adopted NANP pricing and calling a cell phone in American Samoa can cost at least $1.49/minute. Cell phone numbers start with either 7 or 2 and most commonly are 733 or 258 numbers.
The 684 area code came into effect 2 October 2004. Before then, American Samoa did not participate in the NANP, but used the same number (684) as its country code.