No additional charge by the ship. Cell phones are cheaper than ship to shore charges by the ship's communications system. Those can run $8-$10 per minute. So it's all relative.
Ok I see. I was on a cruise one time when I was a child. I didn't even have a cell phone then. I do need a phone for business almost all the time now, I can see how this service, even at high charges, would be very worthwhile looking into.