Setting up a two-step verification (or authentication, depending on the site) service is more or less an imperative when it comes to keeping your online information secure. Services such as Google, Facebook, Twitter, Apple, Dropbox and more have rolled out two-step verification in one form or another.
During the setup process for these services, you are required to provide a phone number where you'll have the option to receive a six-digit code that is required to grant you access to your account. But, as a recent Forbes article pointed out, phones get lost, which can in turn lock you out of your accounts until you've replaced the device. One workaround for this scenario is to use your Google Voice phone number to receive your verification code with. For those who are unfamiliar, Google's Voice service offers anyone with a Google account a free phone number, capable of sending and receiving text messages, as well as placing phone calls with.
It's a convenient solution, but one that has a huge caveat; text messages sent from Facebook or Skype won't be received by Google Voice. Facebook and Skype (only using them as an example because they are listed on this Google Voice support page) send text messages as e-mails, instead of SMS, and Voice doesn't support e-mailed text messages. But, the good news is that it does work with two-step verification on Twitter (with one handicap.)
The lesson here is, be careful when setting up two-step authentication using your Google Voice number. Not all services are supported by Voice, and the last thing you want to do is lock yourself out of your account.
In my testing I have found that both Google and Twitter SMS messages are received by Google Voice, although it's possible that can change in the future.
If you do use Google Voice with Twitter, beware that verification code messages aren't forwarded to your phone number, so you either have to log in using a Google Voice app, through the Web site, or enable the forwarding of text messages to e-mail to view your code. To enable the last option, view your Google Voice account settings, and under the "Voicemail & Text" heading you'll find a check box to enable forwarding; check it.
Fortunately, you should be able to determine if Google Voice and the respective service you're attempting to set up with two-step authentication will play nicely during the setup process. Most services require test messages to be sent to and from the phone number you're adding. If there are any issues during this process, ditch your plans to use Google Voice and go with your real phone number.
Oh, and one more thing -- if you're using your Google Voice number as part of your two-step authentication setup, you really, really should activate two-step authentication on your Google account.