If you’re a long-time Gmail user, you no doubt know about and/or use Google Voice — a free web-based platform that provides users with an online phone number for making calls online via an Android, BlackBerry, and now the iPhone. If you don’t have a Google Voice number set up, you’re missing out on a very useful contact tool. Check out Ryan Dube’s 5 Cool Things You Can Do With Google Voice for more details.
I’ve been using my Google Voice (GV) number primarily as a second contact number for which family and friends can reach me when I’m a way from my phone and working on my computer. I also post the number on trusted website forums and registrations.

For a few months in 2009, a third-party Google Voice app was available for the iPhone, but Apple decided to remove it along with the official Google Voice app that was released a few months later. But now, the official app is back and available for free in the App Store.

How It Works

In order to use GV on your iPhone, you need of course to have a Google Voice account. Once you’re set up, you will be able to use your provided number to make online phone calls, receive online voicemail, and make free calls and text messages to the U.S. and Canada, as well as make low-rate calls everywhere else.
After your account is set up, you can download the free app for your iPhone [iTunes Store link].
The app interface works similarly to the web platform. When you make a phone call using Google Voice, it first rings back the designated number of your iPhone and then it makes a call to the contact you’re trying to reach. You’re not charged by your carrier””in this case, AT&T.
Similarly, when someone wants to contact you using your Google Voice number, your regular iPhone number is called by Google, which in turn reconnects you to the caller if you accept the call. Your GV number is displayed as the caller ID when making calls, so the contact never sees or knows your regular iPhone number.
You can make phone calls from contacts in your iPhone address book or simply enter numbers on the dial pad.

Forwarding Calls

I mostly don’t answer my Google Voice calls immediately. Instead, I have numbers forwarded to me via email. These emails will include not only the voice message, but also a written transcription of the message. The transcriptions are not always accurate, but they’re great for seeing phone numbers spoken in the message.
There are other call forwarding features you can do with Google Voice, but unfortunately you can’t do them on the mobile platform. See Mike Fagan’s article on how to set up call forwarding with Google Voice.

Notifications

If you want to be instantly notified when voicemail messages arrive in your Google Voice inbox, you will want to allow the app to send you push notifications, which means the app doesn’t have to be opened in order for you to receive notifications.
Otherwise, you will have to open the app to find out what new messages were received.

Other Features

While the Google Voice app doesn’t provide you the ability to set your custom voice answer message or filter phone incoming numbers; it does allow you to star important messages, set up quick dialing for specified contacts, and view the history of your phone calls and messages — both received and initiated by you.
Google Voice is pretty easy to use and as long as the service is free, there’s little reason not to set up an account and use it. Hopefully, in future updates, Google Voice will be available for use on the iPod touch and iPad, just as Skype is. It shouldn’t be too difficult to set up the app as a VoIP/SIP dialer.
Also, note, there is a third-party app called GV Mobile+ [App Store link] that sells for $2.99. I have not used that app, nor do the features seem that different.
Let us know how Google Voice works for you, both on the web and via your iPhone or other mobile device.