Some lines have apps that can be used without cost but most incur wifi charges I believe. Being and old hand at a bunch of things. I use the room phone and the phones around the ship to leave mssgs for my travel partners. Others use those "Walkie talkie" radio's but they sometimes have a record of being a bit obnoxious due to loud usage (mostly by kids) Stickies on doors or left in "Secret locations" Most of the time I just wander and let them find me. lol Its my holiday.