You'll need to unlink your existing number, and then re-configure FaceTime with the new phone number.
How to unlink a phone number
To remove a phone number from an Apple ID, sign out of FaceTime and Messages on your iPhone. Sign out in the following locations:
- Settings > Messages > Send & Receive. Tap your Apple ID, then tap Sign Out.
- Settings > FaceTime. Tap your Apple ID, then tap Sign Out.
This should remove your phone number from other devices using the same Apple ID with FaceTime and Messages.
If the phone number is still available on other devices after you sign out of FaceTime and iMessage on the iPhone, you may need to sign out of iMessage and FaceTime on all your devices, then sign in to FaceTime and Messages again on devices you want to use.
Note: If you no longer have access to the iPhone that is using the number you want to remove, reset your Apple ID password.