Sprint is currently the only carrier that integrates Google Voice into its service, although it looks like this may be about to change, as Google has told Cnet that they are looking at partnerships with other carriers.
Vincent Paquet, who is the Group product Manager for Google Voice told Cnet that “We are having discussions with other carriers about this,”, although he didn’t give any specific information on which carriers Google were in talks with.
He also said that their current partnership with Sprint, which lets you turn their handset number into a Google Voice number has been successful, and obviously Google are now looking for other partnerships.
Anyone can currently port their number to Google Voice, but the difference with Sprint’s offering is that it doesn’t include the $20 fee to port the number, and the service can easily be setup online.
Source Cnet, Techmeme, The Verge