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.
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.