You should be able to get "roaming" coverage in most ports. When you are "at sea" there will be no coverage (no land = no cell towers = no service). Watch your roaming charges when you are in port. Knowing wher eyou are going and calling your cell company's customer service should get you all the info you need. Some cell companies require you activate international calling before you use it (prevents theft)
You should get the ships phone number to your parents so that if there is an emergency, they can call and talk to you (or leave a message) that way you are not paying for roaming charges and you only get charged if they need you. But be sure they understand its for emergencies only, the ship to shore charges are nothing to laugh at.
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