Google officially released Google Voice for iPhone, so you can download Google Voice for iPhone for Free. Like we all know there are two apps avaliable on App Store but not officially by Google. This App is not free. Check this site (1, 2) for more info about two apps for Google Voice

Google Voice for iPhone

Such as on Android Device Google Voice offering something useful and wonderful featured such as

• Access your Google Voice account right from your iPhone.
• Receive push notifications for new text or voicemail messages.
• Send free text messages to U.S. numbers and make international calls at cheap rates.
• Listen to your voicemail, read transcripts and manage your Google Voice inbox.
• Display your Google Voice number as caller ID when making calls.
• Call contacts from your iPhone Address Book or enter new numbers on the dialpad.

Please notes, to use Google Voices for iPhone you need to setup Google Voice. You can sign up easily on http://www.google.com/voice and Google Voice is only available in the United States.

Download Google Voice for iPhone here