- Don't use your phone*
- Buy the ship's WiFi package, put your phone in airplane mode then turn WiFi back on. This may not allow calls but should allow texts or iMessages, Facebook, etc..
- When anywhere near the ship, forward all calls to voicemail. Change your voicemail greeting to request the caller send you a text message. If you are careful about doing this, it allows you to receive texts for free but never have to pay for a call. Do not let your phone go unanswered or reject calls, it's not the same. Forward ALL calls to voicemail in the phone's forwarding settings and check to make sure it "took".
*Be sure that your phone is either off or in airplane mode when anywhere within 100' of the ship. The cruise companies are absolutely avaricious and use all sorts of sly techniques to maximize billing for calls, even those you never get. They are not supposed to operate their shipboard cellular systems with 12 miles of the coast, but they do. They keep your phone shown as registered on their system, long after you turned it off. If you have kids on the trip, just confiscate their phones.
thanks for your suggestions.