A lesser company would have given up after 18 months of trying to get an iPhone app approved by Apple. A lesser app would not have gotten any attention after that time. But, Google is not a lesser company. And, Google Voice is a hot ticket service. So, this app hit #1 on day one of its release after an 18 month waiting period.
Google Voice 18.104.22.1686 (iTunes App Store)
Google Voice for iPhone (Google Voice Blog)
Everything is there. Calls can be made and received using a Google Voice phone number. Text messages work too. You can see my first incoming text as an alert over a locked screen on my iPhone in the screenshot here.
Google Voice for iPhone requires an iPhone running iOS 3.1 or newer. And, Google Voice service is currently only available int he U.S.