After turning 2-Step Verification off for a user and change the recovery phone number, when trying to set up 2-Step Verification again you are prompted to verify your identity through the old phone number.
- Admin console
- Login challenge
- 2-Step Verification
- In your Google Admin console.
- Go to Menu > Directory > Users.
- Click the row for the user account to display the user information page.
- Click Security.
- Scroll down to find the Login challenge section, and click on it.
- Click Turn Off For 10 Minutes.
Login challenges are commonly presented when performing major security changes in a Google account.