Since the mid-1990s a few areas, namely US territories in the Pacific Ocean, joined the NANPA. I understand why they did this. The calls became "domestic" for billing purposes. I also imagine it made it a little easier for military personnel and make and receive calls to and from the mainland US (international dialing sometimes confuses Americans).
Later this year Sint Maarten is scheduled to abandon +599 for +1-721. We briefly touched on this here:
Sorry if this seems like an esoteric question. I got to thinking a few days ago when a waste of an area code it is for a country/territory with only a handful of exchanges.