Now you can port an existing mobile phone number to Google Voice for a small fee: $20. The process is rather complicated and may incur additional charges, but it's important to keep in mind that Google is not a wireless carrier (at least not yet) and you'll still have to get a non-Google phone number.
Number porting is a feature in which your existing mobile number will become your Google Voice number and you can take advantage of the full set of the Google Voice features, like one number to ring all of your phones, online voicemail, and more. Number Porting also costs a one-time fee of $20, payable via Google Checkout. Note that you will still need carrier service (Sprint, T-Mobile, Cricket, etc.) to receive calls on your mobile phone.
At the moment, you can make free calls in the US and Canada if you use Gmail Chat, but it's likely that this feature will also be available in Google Voice's mobile apps for Android and iPhone.
{ Thanks, CJ. }