Or even check with your Long Distance carrier about getting a calling card. AT&T is fairly universal in having access numbers in countries. Most can tell you what the rates for each port would be (since you know youw itinerary). Most providers don't charge you for a calling card.
The other thing to consider if you DO buy the international calling per paid cards is if you don't use the entire amount (say $10 is left on it), you can always give it to a crew member at the end of the cruise as an "extra" tip. They can always make use of it.
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, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013