Wifi calling is a great way to preserve batt. at home, esp. if you have a weak cellular reception… don’t forget to put cellular on airplane mode to maximize benefit. Not sure about cruise ships, but I doubt they can cherry pick wifi used for phone service (wifi-calling or skype etc). We use it traveling internationally (land) but keep in mind wifi-calling treats you as if you’re home eg, if in Paris making a local Paris restaurant reservation, using wifi-calling will be treated as a international call. Use Skype for that instead.