How to deregister an old number off my iCloud
How de register a number off my icloud
MacBook Air 11″, macOS 10.14
See if anything here helps.
iMessage on your iPhone or online - Deregister.
You need to change/add the number on your Apple ID. 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.