You can send iMessages to phone numbers or email addresses if the recipient has a device that supports iMessage. Before you can set up your mobile phone for iMessaging, you need to activate your Apple ID.
Tap the indicator next to "iMessage".
Tap OK to turn on the function.
Tap the indicator next to "Send as SMS" to turn the function on or off.
If you turn on the function, iMessages will be sent as SMS if the service is unavailable.