You do not use phone number, another email as a way to verify the Yahoo user because these have made Yahoo is the hacker of Yahoo email.
Yahoo has been used phone number and other email as a way to make Yahoo users to verify themselves while Yahoo never care Yahoo email users could think Yahoo as the hacker of Yahoo email for other gains. The Yahoo email users refused would never log in to see their email. It is the way Yahoo control the Yahoo users.
Could Yahoo please explain how these verification ways work?