Two-factor authentication is growing in popularity as a security measure. For example, Google, Facebook, Twitter, and many other services all have two-step security options today as well as many banks and credit unions.

I'm wondering whether using a Google Voice telephone number as the so called Physical device to which one would receive text message codes to confirm one's possession of the Physical device is a mistake since potentially your GV account could be hacked, and then your Physical device is effectively stolen?

What if you are using two-factor Google authentication which puts an application on your smart phone to confirm identity, does that make using GV for others safe if you assume Google is safe?