Assuming your phone is authorised for overseas use the yes if you are near land. Some modern ships (P&O) auto link your mobile to satellite when away from land. Have used my mobile in mid Atlantic.
Only when you are near land, I have used my mobile in the Med but signal is lost as you sail out to sea however some of the itineries stay close enough to land to get a signal- e.g along Spanish coast to Gibralta.
We were on princess aug 07 - i was able to use my Vodafone (cheap) GMS phone on every occasion - in port and at sea with exceptional service. Marvellous!. Ship's phone charges via their phone is about $5+/minute!!