New VOIP option for GV - single app solution: GrooVe IP
So I have not tested this yet, but there is a new app that supposedly makes calls via Google Voice and also receives incoming calls. No external web accounts needed. No extra phone numbers. Just download the Groove IP app, put in your Google Voice account details, and you are good to go.
Price increase: $3.99
Official Groove IP Thread: [APP] GrooVe IP - Google Voice VoIP - Android Forums
Okay, it seems the issue is that if you have gmail logged on computer and if the call gets forwarded to it. Then it doesn't come to the app on the phone.
I though that incoming calls would be forwarded to all logged in instance, but apparently it is not so. So you need to make sure to log out at other places.