FreeTalk

f you have been waiting for fast and intutive google voice client application, then your wait is over. FreeTalk is a client application for goggle voice server (https://www.google.com/voice) and let you make free/cheap local and long distance calls. In additional, send and receive free sms without increasing your monthly phone bill. FreeTalk gives you the overview of you recent messages on your start page so that you don't have to drill down any further if you don't need to. If you want to make a quick call, its right at your start screen. Try out the free trial version which lets you even make calls and send sms.

FreeTalk features -
1. Windows Phone Panorama home screen to show your recent history, quick dial.
2. Intelligent refresh to avoid unnecessary updates.
3. Interactive SMS. You don't have to refresh to see your incoming messages. Your SMS screen gets updated as the messages arrive.
4. Auto refresh to keep you upto date with your messages and voicemails.
5. Play voicemail locally on the device with transcript.

and a very unique feature, bing image of the day directly on your home screen. This gives your free talk application a refreshing look every day.

Free talk supports push notifications using gvmax.

more info - http://ashterisk.wordpress.com/freetalk-a-google-voice-client-for-windows-phone-7/