I moved back to Vietnam and canceled my Korean sim card already but I forgot to deregister it on iMessage. Therefore, I cannot have that Korean sim card to put it back to deregister that phone number on my iMessage. What should I do to remove it on iMess?
You need to remove the number on your Apple ID. You may have to add the new number before you can remove the old number. After the change, allow up to 24 hours for the change to sync. Before trying to make the change, sign out of Messages/FaceTime on all devices/computers.
But I do not have sim card and no longer use that phone number to get the code to deregister
I have this exact same problem. You cannot deregister online without the original sim card. I don't want someone else getting that sim card and all my old messages which are personal.
Deregister phone number on iMessage without a sim card
