Facetime, Skype, WhatsApp, etc all have free or cheap phone service with wifi.
I was shocked recently on a transatlantic flight that the whatsapp phone rang midway through the trip. United has global wifi and they had not as of then eliminated the access to voice apps.
If you don't want to rent a phone, you can buy one at a discount store in the States and get a SIM once you are in St. Maarten.
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