iMessage
Why isn’t my iMessage not working on the new phone
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Do this, and do it again if your number is ever changed in future.
Settings, Messages, Turn off iMessage
Settings, Face Time, Turn off Face Time
Restart the phone.
Settings, Messages, Turn on iMessage, SMS, MMS
Settings, Messages, send & Receive, Start new conversations from, select your Apple ID which should now be there.
Settings, Face Time, Turn on Face Time
If you still can't send or receive messages on your iPhone, iPad, or iPod touch read this:
If you can't send or receive messages on your iPhone or iPad - Apple Support
If you have activation problems see here
See If you can't turn on or sign in to iMessage or FaceTime – Apple Support (UK)