Finally, number porting is available for all existing Google Voice users, not just certain ones. This means anyone can make the mobile number you’ve always used turn into your Google Voice number. Why would you do this? So you can make your phone number ring any device you want basically.
Take a look at the video Google has posted on Youtube.
How to port your number is simple. To get started, log in to your Google Voice account, visit the Settings page and click on “Change / Port” next to your Google Voice number. Remember, porting a number to Google Voice will terminate your contract with your carrier if you are in one and may expose you to early termination fees/suspension of your phone. Porting your number could take a couple to a few days and there is a $20 fee associated with porting to Google.
Source: Google & Androinica