Here is video from google voice
Step1. Login google voice and Go to Settings.
Step2. Click the Call Widgets tab.
Step3. Click the Add a new Call Widget link.
Step4. Select the appropriate settings for your widget.
Step5. Click the Save Changes. Then, copy and paste the code in the 'Embed' field into your webpage, your blog, your auction or networking site, or anywhere you want.
PS: It is free within the continental US. All other calls are charged at the rates shown here(https://www.google.com/voice/rates).
Enjoy the hi-tech life and have fun.