Check out this website I found at macosxhints.com
There has been a lot of controversy concerning Google Voice and the iPhone. Here is an AppleScript that allows you to place Google Voice calls directly from your iPhone’s Contacts/Phone app.
The script works by adding numbers to contacts in Address Book that dial your contact via Google Voice. The new number is prefixed with your Google Voice number, your pin, and the number two. It dials into your Google Voice number and places the call to your contact.
