My experiences with AT&T digital one rate and a Nokia 5160? phone is that you can add international calling as an option for a $5.95 hookup and $5.95 a month thereafter.
Calls are then 1 to 4 bucks a minute based upon where you are.
We had service in Cozymel, Ocho Rios, Grand Cayman, and Plaza Del Carman on our last trip. No service on the high seas but around the ports we had a really good signal.
Worked good, great peace of mind with the kids at grandparents.
Hope this helps
Doug