To get started, make sure to install the voice and video chat plug-ins. Sign in to Chat, click on the “Call Phone” link, and a “Call” window will open. You’re all set to go!
I noticed this option: If you use Google Voice, you can enable Google Chat for forwarding and receiving phone calls in Gmail!
Phone calls in Gmail are a feature available only in the United States as of December 2010. If you don’t see the “Call Phone” link at the top of your Chat list, and you’re in the U.S.A., check and make sure that your Gmail language is set to English (US).
For complete details, including a 40-second YouTube demo video, see Phone calls in Gmail – Gmail Help.