VOS, like other similar cruise ships have in-room voice mail system, call & leave a message (plus leave sticky's as suggested)

Designate a centralized public meeting place for everyone @ a preset time if lost/unable to get in touch (nobody in the cabin, etc.)

Depending on your carrier/mobile plans - texting/SMS can be almost free or inexpensive to receive messages, and typically no more than 25 cents or 50 cents (USD) to send a 160 character text.

If you are staying in a suite, penthouse or similar higher tier, comp. use of ship-only (Nokia-style) candybar phone are often included - check with your cruise planner or TA.

Walkie-Talkies are nice gadgets but if they work so well, ship officers & crews would all be carrying & using them, right ... The select few are issued candybar phones exclusive to the ship's network and everyone else use the ship's hard-wired telephone system & extension phones found everywhere. Your typical UHF WT's are point-to-point communications ideally suited for unobstructed outdoor line-of-sights use, but you are free to believe otherwise. Keep in mind that at foreign ports-of-calls, use of devices might be restricted or prohibited by local regulations. Lastly, buy plenty of batteries and/or be prepared to do a lot recharging - they are very expensive onboard.