F-Mattox already gave you the best advice. But I'll add another option, if you don't have to "phone home" but could use email to check in, You could buy a block of internet time and email back and forth.
But check with your cell provider and they can give you rates for each port. Or use a calling card. Those are nice also, because if you buy a larger prepaid calling card than you think you will need, you can give the "unused" portion as a tip to your steward. They use them to call home when in port.
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