[size=x-large]NO - NOT FREE!!!![/size]
When you place a cell phone call, your phone sends out it's signal to the nearest cell antenna tower (based on signal strength).
If you're on the ship, that tower will be the ship's tower and you will pay whatever roaming and connect time rates the ship charges. Basically, you can figure on anywhere between $5.00-19.95 per minute, or any portion thereof, plus satellite rates (because the ship bounces your call from their server to a satellite, then back to shore).
If you're in port, the same thing applies. Your phone will connect to the nearest tower and you will pay whatever that provider charges, plus roaming, plus international rates!
Oh, and all of this will be on top of whatever your provider charges for your normal connect time (per minute, per call, etc.).
Now, is your call really that important?
You're on vacation. Turn it off.