Ask HN : Question about Google Voice

I was thinking how Google Voice service works. When we sign up with GV, the user gets a dedicated phone No. for free. So, does Google reserve one phone No. for each user? How is Google able to procure so many phone No.s? Some more thought process gave me more hints like : they might have partnered with mobile service providers. Is that how it works?

Also, I know little bit of IP networking. So I can make out how users requests from Web browsers reach over to webserver(Browsers>DNS server>Webserver(or respective servers) etc). Similarly, in VoIP networking space, can anyone explain how this really works? When a user calls a phone No., that is assigned by GV, where does the call go first and how it gets routed?

After little bit of digging out, found that GV is based on Grandcentral. Searched a bit on the net as to if there is any documentation about the Grandcentral. I could not find any.