Sint Maarten is a Caribbean territory known as a Constituent Country of the Kingdom of the Netherlands. Sint Maarten along with 5 more islands (Aruba, Bonaire, Curacao, Saba and Sint Eustatius) used to constitute the Netherlands Antilles until October 2010 when the country was disolved. The southern part of Sint Maarten island (also known as Saint Martin) now uses the North American Numbering Plan and has been assigned area code 721 introduced on September 30, 2011. Before the aforementioned date Sint Maarten had used country code 599 which remains in service for Bonaire, Curacao, Saba and Sint Eustatius. The old and new numbering systems will run in parallel for a year (people using country code 599 to dial to Sint Maarten will hear a message announcing the new area code).
Note that the northern part of the island, known as Saint Martin is a French Overseas Collectivity and uses country code 590.