I wouldn't give Google Voice full credit for innovation on this. The real credit needs to go Asterisk for innovation in the VoIP front, their the ones who brought this tech from out of no where to the masses first...

To Google's credit, they did take something that was useful and made it available for the masses in a super easy fasion and made it better, like searching and email.

Google Voice was originally GrandCentral so its founders Walker and Paquet would seem more deserving of your award of partial credit.

Google has added some features but I think their main contribution can come from popularizing voip and forcing the mobile networks to accept reality.

Twilio is possibly a more ground-breaking service, or at least it will be when they finally support international calls properly.

duh... FollowMe, MeetMe, VoiceMail, Conference, IVR, etc... Asterisk has been doing all those milestones for a decade now...