Apple’s iMessage service can be very useful if you’re on a limited text messaging plan, since iMessages travel over the Internet, using your cellular data plan, rather than being charged at exorbitant per-message rates. When composing a text message, the iPhone Messages app looks up the recipient address to determine whether an iMessage or standard SMS message will be sent, and indicates this by the color of the Send button—blue for iMessage, green for SMS.
Your iPhone will send messages via iMessage whenever possible, but will automatically “fall back” to sending a standard SMS message whenever the recipient can’t be reached via iMessage, such if they are out of data coverage. Fortunately there is a way to disable this feature, since it can otherwise result in unexpected SMS charges: Simply go into your iPhone Settings app, selecting Messages and toggle the option to “Send as SMS” to OFF.
This will not prevent you from specifically sending SMS messages to folks who aren’t using iMessage at all, such as non-iPhone users, but will at least prevent your iPhone from unexpectedly racking up SMS charges when an iMessage can’t be used; instead the iMessage will just remain queued up until it can be delivered via the iMessage network..