Most ships now have a link so your mobile phone will work, ceratinly my quad band one does but I keep it just for quick text messages as the voice option is not cheap, but useful in an emergency.
Yes, you can use walkie talkies. Or there are Dect phones you can hire - believe it is $25. A simple postit note system on the door of the cabin is also a good way to leave messages for where to be at what time. I suggest you sign up for the meet and mingle gathering - a good way to meet your fellow cruisers and they will have tips to share.
Yes you can the yanks use them all the time they seem to work ok.you can also use the cabin phone or the phones dotted round the ship to phone different cabins.
Get your tickets for the ice show ASAP and get there early as the best seats soon go .Enjoy your cruise?
O2 and orange phones have no signal most of the time,however I was on the Indy last week and found we never lost each other,only when coming back from onshore was the post it note system used to let each other know where we were on ship.
Maybe losing each other is a good thing, you get to explore and do things you want to do, on our last cruise we saw americans with walkie talkies all the time, but they only used them to get some one in the front of the queue to buy them drinks LOL.