Authenticator

I'm afraid if my phone get stolen, both Authenticator app and the phone number is lost...

Then just revoke the Authenticator premissions and generate a new one :grin: You can let a PC etc be marked as 'trusted' so they don't need to use 2FA, handy if you have a device which you don't roam about with.

Also you can get codes via email.
 
That means the authenticator app is not sync'd with your phone. You'll need to go back to your account settings and add the device like a new one.