How to deregister a phone number you no longer have?
I need to remove a phone number from iMessages. I no longer have that number or access to it.
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You can edit or change your trusted phone number for your Apple ID account by using the following steps which Apple has in their support document Two-factor authentication for Apple ID
Update your trusted phone number on your Apple ID account page
- Go to appleid.apple.com
- Sign in with your Apple ID.
- Go to the Security section and click Edit.
If you want to add a phone number, click Add a Trusted Phone Number and enter the phone number. Choose to verify the number with a text message or automated phone call, and click Continue. To remove a trusted phone number, click 
next to the phone number you want to remove.