I question what you say because it is not the solution, it may work in a very limited amount of cases if everything is registered properly and just the number got stuck, but most cases it won't work...
Even the solution I posted which is recommended by Apple does not always work that is why Apple says in that solution..."If you still need help, change your Apple ID password and repeat the setup steps. If that doesn't work, contact Apple Support."
Another solution which only works sometimes is just changing your Apple ID password, cause when you change your password it reregisters your phone number, and when it does work you will get a popup from Apple saying your # was registered.
See this post...https://discussions.apple.com/thread/251414717...the user changed the # the way you said and it did not work...
The user above says this in his post..."Given in mind I have changed my reachable phone number in the Apple ID to the new one."