GV Mobile is avaliable for free in Cydia
Yesterday Apple blocked an official Google Voice app from the App Store; meanwhile, they also kicked out unofficial versions, f.e. GV Mobile. But you can still get GV Mobile for free... via Cydia.
The developer of GV Mobile has decided to go ahead and release GV Mobile for free rather than let his work go to waste. You have to jailbreak iPhone to get this app.