the dect phone .. at least on RCCL ships . . is a closed system. You get a phone as part of your job/position. Most cruise ships have a cell tower that anyone has access to . . at a price. The company Oceanphone (and I'm sure others) sells phone service from guest/crew cabins and via phone cards in crew area or now sim cards for phones (that is accessed via a hard wired ship phone or cell tower)
Yaka - if nothing's changed at NCL . . you won't be able to use your phone (and I'd really not recommend it for reasons Bruce mentioned) at least in guest areas.
When you go to work on ships, you kinda go into a culture/communication bubble . .you'll understand once you're on