The country code for Tonga is 676; there are no local area codes. Dial 913 for the international operator and 910 for directory enquiry.
There are public phones throughout Tonga (buy a phonecard to make calls), but a mobile (cell) phone is a more reliable bet. Most foreign phones set up for global roaming work here and coverage is reasonably good throughout the islands.
The two telecommunication companies are Tonga Communications Corporation and Digicel; a cheap local phone, including SIM, will cost you around T$60. Digicel also offers a T$20 Visitor SIM that expires after 30 days.