Unless the ship has something like a satellite system set up you cant usually get a phone signal at sea, but I would check with Fred Olsen - we just switch our mobiles on whenever we arrive at a port and can then pick up a signal. Not too expensive if you both have english mobiles.

Im Vodafone and have never had a problem ringing from sea.
Plenty of Satalites up in space.

The following is from the Fred Olsen web site

http://www.fredolsencruises.com/About-Cruising/Before-You-Cruise/Using-your-mobile-phone-at-sea/Using-your-mobile-phone-at-sea

Not a problem - apart from the expense. We were off the coast of Australia when I received a text from Facebook stating I had a 'friend request'! Needless to say I ignored it until we got home.

Check the international roaming charges for your service provider to prevent any nasty surprises with the cost.

luci

Thomson ships have their own roaming facility I know that so you can use whilst at sea, however the charges are far in excess of your own provider, I turned off whilst at sea.

I guess it's best to just turn on if we actually need to. It will probably be my wife making sure her mother and father are fine and well.

BTW do some ships have Wi-Fi that you can log into with a mobile ?

Thomson do don't know about others will probably tell you on other curise websites.

My advice is only turn the phone on if you actually need to use it.

If you have an internet enabled phone it sends packet data you are unware of and this can cost a fortune - for example to check any external email addresses you have linked to the phone it needs to connect to the mail servers - same for facebook, twitter etc etc.

Wait until you are in port and use a local provider.