The problem I have with Google's authentication flow (and also Microsoft's) is that there's no right way to use it. They both make unpredictable demands at unpredictable times, so you can't rely on being able to authenticate when you need to.
Randomly losing account availability like this is completely unacceptable for critical services like the ones they provide.
In my case, nobody owned the number so our phone provider gave it to us, you can also ask the current owner if they could help, though alarm bells may go off in their head
Shame on you google.