Google Voice works the following way: Any call initiated through Google Voice necessarily has to pass through the PSTN, the traditional phone system. But the PSTN doesn’t do all the work. The call is then handed to the Google space on the Internet, which is where the ‘numbers are pooled’. Say the call is directed to another Google Voice number, that number is identified within Google’s numbers, and from there, the call is sent to its final destination.
We need to keep in mind that the main aim of Google Voice is to unify communication channels, more so than saving on cost. As a result, you can easily switch carrier without having to change phone number, as one number can ring any phone through any carrier.
Cost-wise, this also implies that you still have to pay your phone or wireless carrier, because finally, Google Voice is not a complete alternative to the services of these carriers, unlike Skype and the like.