Many months back Paul Marks and I looked into how to integrate Google Voice and Asterisk. Inbound calling was simple as you direct your Google Voice account to a Gizmo5 number and Asterisk integrates with the open SIP standard that Gizmo5 uses.
However, our task was to be able to make outbound calls without the need to use the web interface to place the call. What essentially had to happen is when you dialed a number on your SIP phone, Asterisk would have to talk to the Google Voice site and handle the call setup for you.
The Cisco 7970G IP Phone is by far one of the nicest VoIP Phones i have ever used. However, getting it to work outside the standard Cisco Call Manager environment with Asterisk can be a challenge. After weeks of testing various configurations and tweaking settings on both the phone and Asterisk, i was able to finally get a working configuration that works for both local network connections and NAT as well.