You can check with your cellular service as well. We have cingular and I always add the World Travel International Feature before we travel. It costs me $5.95 for a month and I can make calls from my cell while in port. The per minute cost is still high, but much less than calling without this service.
Since we had a death in our family while on a cruise and could not get the phone in Mexico to work, I have always done this. I also set up an internet account on board the first day while they have discounts. I log on just long enough to check my important e-mails and answer the ones I need to answer, because you are charged by the minute.