try the phone in a port. Probably won't work at sea. I have heard others say phone cards may not work in other countries either. If it is real important, bite the bullet and pay the 9 dollars but don't long.
My phone company (cingular) told me that I needed to call before I left for my cruise and get my phone "unlocked" for international calls. I don't know if I'll get reception on board or not, but I suspect I will be able to use my cell from a POC.
I don't believe it has to do with the specific phone you have. The service you have can tell you if it will work where. I have Verizon and they said mine will work in PR and the virgin islands but not in Mexico (we're going to Cozumel and GC) so I won't be able to use it when we get there! That sucks.