Google has more than 175 million users, and the service isn't even available to all users yet. Google started rolling it out in phases on Wednesday.

Google's service allows users to make telephone calls from the Gmail interface. Calls to the U.S. and Canada will be free for at least the rest of the year. Google promised to add international calling at low rates in the near future.

"A good deal of those million phone calls were from folks who were just testing out the feature to see if it works," said Michael Gartenberg, partner at Altimeter Group. "The question is, will they continue to use it? I think the answer is yes."

Google's Social Hub

Gmail users have been chatting through PC microphones and speakers via a service called Google Talk since 2005. But until now Gmail users couldn't dial into landlines. Both Gmail users had to be sitting behind their computers and signed into Gmail to talk.

"This shows how Google is looking to build a set of services that become a hub of social activity, and one of the social activities we forget is the good old telephone," Gartenberg said. "By integrating telephony into the overall communications experience, Google has given users one more reason to stay with Google services like Gmail and Google Voice."

Google has designed a user-friendly interface for the service. Users dial a phone number in much the same way they would a normal telephone, but with a mouse instead of fingers. When users click "Call phone" at the top of their chat list and dial a number or enter a contact's name, it rings the number. If the users have a Google Voice phone number, calls from Gmail will display that number as the outbound caller ID. Users can also choose to receive calls to this number right inside Gmail.

Integrating with Android

With the new service, Google is competing with Skype. But Skype still has the advantage with international calling and integration onto mobile Relevant Products/Services devices. Could Google take its new service one step further and integrate it into the Android operating system? Gartenberg thinks it's a logical step, especially if Google plans to become a force to be reckoned with in the Voice over Internet Protocol world.

"Mobile calls today are very cheap. There is almost no advantage to using a Google service like this on a mobile phone," Gartenberg said. "Minutes are not nearly as precious as they were before, with many people being on unlimited or nearly unlimited plans. Nevertheless, it makes sense for them to integrate this functionality and tie more of that functionality into Google Voice."