My AT&T phone worked while we were in San Juan (emergency stop for a medical discharge) and St Thomas. I didn't try using it at St Maarten. Puerto Rico and the USVI are not included in the Digital One Rate plan... I was charged about $0.75/minute for roaming and long-distance. Still a lot cheaper than using the ship's phone. If your phone works at all at St Maarten, you'll be subject to whatever rates the local company there chooses to charge.
As for people calling you onboard, from the United States, your party can dial toll-free... 888-DC-AT-SEA (888-322-8732). If the party calling you is outside the United States, they'll have to dial direct +1-732-335-3281. They will need to know which ship you're on and your cabin number.
Make sure this is used for emergencies only, because you will be charged somewhere between $7.50 and $9.50 per minute for the incoming call.
A cheaper option is email. Create a Hotmail.com account to use while you're on the cruise. Give the address to those folks you want to keep up with on a daily basis or so. Your cost will be $0.75/minute.