In the fourth quarter of 2010 gmail launched its newest and most attractive feature “Call Phone” which enable a gmail user to call to any land-line/mobile phone in US and Canada for free (introductory offer). But many of the users just cannot see this “Call Phone” option at all. It looks something like this:

• To enable this feature you must have audio/video plugin installed on your computer.
• To install these plugins click on a friend in chat window bar.
• In the pop up chat window on the top there is a video and audio symbol click on that and it will install gmail audio and video plugins on your computer.

• You might not be able to see the “call phone” option yet. There is one simple reason for that. This feature is available to the users of US only. That means the default language of your gmail account should be English(US).
• To set the default languageclick on the “setting” button on the top right of the screen in your gmail window.
• It will take you to setting page where look for the “language” option and choose “gmail display language” English(US)
• Save the setting and check your chat window bar. There it is. Happy calling.

Sources: http://www.google.com/support/forum/p/gmail/thread?tid=206e1af8d16ee5fe&hl=en

http://gmailblog.blogspot.com/2010/12/free-calling-in-gmail-extended-through.html