To use the service, Gmail users need to adjust their Language settings to "English (US)" if it is set to any of the other options. After this setting is changed you should see a "Call Phone" under your name in the Chat fragment on the left-hand side of the screen. Clicking on Call Phone will prompt you to download and install a browser plug-in, after which you should be ready to start making calls. People using Google Voice can call numbers in the US and Canada for free until the end of the year, and other international numbers have calling rates as low as US$0.02 per minute.
This backdoor will work for making calls through Google Voice, but it won't work for people hoping to receive calls through the service. This requires the user to set up a Google Voice calling number, an option not available to users outside of the US. If you need any help setting up the service, check out the video above from our colleagues at CNET US.
Simply change this language setting to "English - US".