There are already some applications in the App Store that enables Google Voice such as GV Mobile+ and GV Connect, but now Google drops their official Google Voice app. To use this app you need to sign up for a Google Voice account, which you can only use in the United States. Google Voice is available for free in the app store.

• 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.