Unfortunately, the number porting option was only available for some users for a short period of time and is currently in testing. As you can see in the photo above, the cost of porting the number is $20, in addition to the possible and likely expensive early termination fee that you would have to pay with your cellular provider.
Engadget noted in an update to the original story that the feature disappeared, but that it may appear again at any time.
Google Voice is an app which allows users to make and receive calls, replacing the cellular providers. The app is seen by many as a smart choice because of the pricing scheme for phone calls, as well as several other useful features. Unfortunately for many, Google Voice is currently only available for US residents. Engadget added that Google plans on offering the number port service "to all users in the near future."
Image from Engadget