Q: I travel a lot and use iMessage regularly.

I travel alot and use imessage regularly. From previous experience of being charged for a text message, I know I need to put mms and sms to the off postion.  I also shut down the 3G and data roaming. The only thing on is wifi. I was in the USA, in a wifi zone, everthing off except my wifi, sent a message through imessage to another iphone and it turned green and was sent as a text. Not sure how this can happen. I called my carrier, they removed the charges but they said Apple should be the one refunding me. Is there some other setting that I am not aware of ? I feel that the only other option I can turn off would be the phone itself.  How is it possible to still be sent as a text when all settings are off ? Any feedback would be appreciated... thanks