How to de register the Apple ID phone number

If you want to deregister it from your Apple ID, you need to change/add the number on your Apple ID. Look in the Security section when signed in online. 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.

Apple ID - Manage

Sign into appleid.apple.com and go to account security. There you should see your trusted numbers. Add your new trusted number first, then delete the old one. Also you can hit the drop down arrow by sign out at the top and go to personal information and see the reachable at settings. Change your number there too if you aren’t using that number ever again.