In November 2009 google acquired Gizmo5, a famous VOIP telephony service that allows users to make and receive phone calls right from their computer and mobile phone devices. The recent announcement in Gizmo5 states that the company is shutting down its operation and refund of available credits in users account also possible now.
As of April 3, 2011, Gizmo5 service will be discontinued. If you are an existing Gizmo5 user, please log in to your account to learn more about available options for call credit refunds, credit balance transfers and the possibility of keeping your call-in number.
You can make phone calls from gmail to mobile for free, where google deployed this technology after acquiring Gizmo5. However, gizmo5 is more than gmail free call feature. Gizmo5 allows you to make and receive phone calls using VOIP Telephony service.
Google was working to bring a desktop client to make and receive phone calls based on Gizmo5 technology. But that project got changed to Gmail/Gtalk browser plugin.