If you have an unlocked phone that you use at home, you can purchase a Chippie sim card and swap it out on the island. The best place to do that is at Tech Hub in the Carrefour store at the roundabout in Philipsburg (next to Mark's). WiFi is great, but if you're in the car and have an emergency, you can't make a call. And WiFi on the island isn't very reliable in many places.