This past week, Google released the integration of GMail and Google Voice which allows GMail users make free phone calls to anywhere in the US and Canada. At this time, the service is only available to US and Canadian users. Making international calls however, is subject to low rates which can be purchased online.
With GMail Google Voice, users can now make and receive free phone calls to any landline or wireless phones in the US and Canada until the remainder of the year. Google plans to make this service free as long as international calls can support the service. To sign up for Google Voice, go to http://voice.google.com.
You will be asked to choose a local telephone number from a list of numbers available. Once you secured a number, you will need to open up GMail to activate Google Voice. Click Settings -> Labs and enable the Google Voice module. To make a call within GMail, you will need to login to Google Chat.
The green phone will light up underneath your user name when it is ready. Dial a number and make your free phone call. GMail Google Voice works best with a headset with a built-in microphone. Enjoy.
Finally, here a video tutorial on how to make calls in GMail Google Voice.