Trusted Apple ID number
How do I change my trusted phone number if I don’t know the trusted phone number?
iPhone 8, iOS 14
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You need to change/add the number on your Apple ID. 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.
See if anything here helps. If necessary, you can use account recovery as explained in the link at bottom of the article.
Verification code - how to get one and sign in with two-factor authentication.