Why is signing up with a mobile number an issue? Where I live, you can buy cards freely l, so I could just use a throwaway card to sign up, and use it only when I need to sign in/restore account.
In most countries (eg Europe, US, Brazil, China, etc.) it's not the case and you have to give your ID (or some sort of identifier linked to your real identity) to get a SIM card or any internet service
Now imagine if you could do that without having to “use a throwaway card to sign up, and use it only when I need to sign in/restore account”