It's possible to add a phone number to your account which will be used for account recovery, if you have forgotten your password and no longer have access to your registered email address. Your number won't be used for any other purposes.
After signing in you'll be taken to this screen, where you'll need to:
An SMS will be sent to your phone with a 6-digit verification code.
You'll be taken to your account profile screen which now displays your linked number.
You can use the buttons at the bottom right to:
your account recovery number.
Click on 'Log out' at the top right and close the browser window to finish.