@Dunkman wrote:iMessage needs data to send messages effectively to other iphone users (when not using Wifi). You may need to go to apple servers to logout out of iMessages completely. Apple servers check if you or somebody else is using iMessages and if they are registered, will only send iMessage if on wifi or data. Easiest to keep data on as iMessages does not consume lots of data.
@mattsregister, @Camera4617 is correct. It should just be a matter of disabling iMessage -- if you're okay with losing the benefits of iMessage (you'll lose the ability to send/receive read receipts, share location, message others using their e-mail, etc.).
First, go into Messages (Settings --> Messages) and view the "Send & Receive" under iMessage (if you've disabled iMessage, temporarily enable it, and wait for activation to complete; you'll then see the option).
You'll then see how iMessage senders can reach you, and how your outgoing messages are initiated (ideally from your phone number, and not your e-mail/AppleID). This step is just to give yourself context about your recent conversation threads -- i.e. if new conversations were initiated from your e-mail address, that would explain any attempts to "resume" the thread being in iMessage.
Now, go back to Messages, disable iMessage, and if necessary, start a new thread with the recipient phone number (rather than resuming a previous thread, especially if you had the iPhone set to start conversations from an e-mail). The "send" button should now be green and send immediately (even in the absence of WiFi/data), and responses should shift over to SMS (first one may take a few minutes, if the recipient previously corresponded with you via iMessage). If this is the desired way to communicate, leave the iMessage setting off (don't flip it on and off, because this will confuse other iPhones as may they "think" you're a valid iMessage recipient, which will delay delivery).
Note that if you're in the habit of turning off cellular data, MMS (photos and videos) will not come through until you re-enable cellular data (even though these do not consume your allotment of data).
Having said all that, I agree with @Dunkman in that keeping data on for iMessages provides a better overall experience, and that text messages via iMessage consumes very little data. Of course, that changes if you send/receive pictures or videos via iMessage (whereas using MMS does not consume your bucket of data), so that's obviously a consideration, depending on how you interact with others.
I have an iPhone with no data plan, but I've managed to stretch a 1GB add-on over more than a year, even with iMessage enabled all the time, and data always turned on.
