Deregister phonenumber on I message doesn’t work
Impossible to deregister my old phone number. Have a new number but the old stay in Apple ID
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Have you tried the steps in this article?
iMessage on your iPhone or online - Deregister
Have you tried signing out of Messages/FaceTime after doing the above and then sign back in?
Have you tried signing out of Messages/FaceTime and then change the number on your Apple ID? Allow up to 24 hours for the change to sync. Then sign into Messages/FaceTime and test.