Unlike Google Voice and Google Talk (ya I know, it gets confusing keeping these services straight) which required both users to be sitting at a PC and logged into Google accounts, this service enables users to call anyone on their mobile directly from within Gmail. According to the Google blog, calls to the US and Canada will remain free, at least for the remainder of the year – calls to the U.K., France, Germany, China, Japan—and many more countries—for as little as $0.02 per minute.
If you have a Google Voice phone number, calls made from Gmail will display this number as the outbound caller ID. And if you decide to, you can receive calls made to this number right inside Gmail (see instructions). For more information, visit gmail.com/call.