Google has briefly trialled number porting for its Google Voice service, offering some users of the VoIP system the chance to bring over their existing mobile number rather than use the Google-supplied number. Priced at $20, Google has confirmed that it plans "to offer this feature to all users in the near future."
"We’re continually testing new features to enhance the user experience. For a limited amount of time, we’re making the Google Voice number porting process available to users. We don’t have any additional details to share at this time, but plan to offer this feature to all users in the near future." Google statement
The porting process - which has now reportedly been pulled from users' options pages - basically involves validating your number with Google and then agreeing to the terms & conditions. Those include the expected warnings about potential early-termination fees from your existing carrier, which may be imposed if you try to bring your number over to Google Voice before your existing agreement is complete.
Google Voice allows users to route incoming calls to multiple phones simultaneously, so that you can have, say, your landline and your cellphone ring at the same time and simply answer on whichever is nearest. With the appropriate apps for Android and iOS, you can also send and receive SMS messages.