You can call and text just turn the data setting to off. You won't have visual voicemail, email, nor the ability to send mms (multimedia messaging service, picture texts). We have ATT and have done this on at least 5
dcl cruises, trips to Europe and Asia, no weird charges beyond the texts and calls we made/received.
Also buying an international package for some voice or texting is okay but keep in mind that with cellular at Sea that is a different animal than the texting or voice on land. Since you are using the ship based towers the texts or calls you make are not using up your international plan. For example you can buy a small package of 50 texts for a set price from ATT but Cellular at Sea texting is still an extra charge. The same with voice, whether or not you have the international service add on the Cellular at Sea service is $2.49.
As to using wifi for texting, its kind of a waste of money, especially if your goal is to text to other iphones on the ship. You would be using the minutes twice as fast, and wifi minutes may be more expensive than the actual texting charges. Sometimes wifi signal is spotty and sometimes it won't connect so just more wasted time. Unfortunately cellular signal is spotty at times too, depending on where you are on the ship remember you are traveling on a 100,000 ton steel cage. Texting on wave phones is just like texting before having the full alpha numeric keyboard at your finger tips.
No international dialing code for texting, there may be an option in the settings for automatic help with international dialing and I pretty sure I have this on so this step may be transparent because of it.