Well, personnelly, I'd just use my AT&T card. They have access numbers in most every country and then you don't have to worry about having enough money on the pre paid cards.
Call your long distance provider and ask them if they have a similar calling card if not AT&T. I have not used the international cards.
IF you do go that route, an idea was brought up on her some time back that if you buy a bigger cash value calling card, and don't use all the minutes on it, you can give it to your room steward our waiter as a "tip" because they call home from those ports almost every cruise and to be able to do it for "free" would be a nice thing.
If you want to use your cell phone, again you'll have to call your provider because not all phones can do international roaming. If you tell them where you are going and what phone you have, they call "activate" international roaming on it if possible.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013