This option causes the most confusion and the reason is that if you and five of your friends all have iPhones, you can “text” each other as if nothing ever changed. However, you aren’t actually texting; you’re just using Apple’s messaging service to send correspondence back and forth over a Wi-Fi connection. Therefore, you can only converse with others who have Apple products.
With iMessage, you can still receive messages abroad while data is turned off. And if you’re in Airplane Mode but have Wi-Fi set to “on,” then you can receive iMessages which appear like text messages but they will be in blue versus green. Ultimately, you will not receive the green message when you are abroad and your cell phone data is turned off. So, if you have data turned on and you see green messages – look out! You may be getting charged insane rates. Remember, Airplane Mode – on; Wi-Fi– on.