re: Apple ID, dropping phone number for separate apple ID

On any devices that used old ID and had wife’s phone number pad an option you need to make sure it’s removed and signed out in messages, facetime, and as a trusted number for old ID (add new one first). Deactivate both phone numbers under iMessage on each specific iPhone before reactivating number first and then activate with Apple ID.

You might be running into the below for a new account. Not sure if you can add some verifications within new AppleID online to quicken this up if this is issue.

quote:

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If you can't change your Apple ID

If you try to change your Apple ID to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later.

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https://support.apple.com/en-us/109353

If her number will not get removed from you Apple ID after signing out and removing it everywhere you might try the deregistering process for moving number away from an iPhone. I have never used this before, but I have heard others that had to use it.
https://selfsolve.apple.com/deregister-imessage/