http://www.google.com/support/chat/b...?answer=187936
Receiving a phone call
In addition to calling a phone number in Gmail, you can also receive calls if you have a Google Voice account. If you already have a Google Voice account, it must use the same username as the Gmail account you wish to receive calls in. If you aren't a Google Voice user, yet, you can upgrade for free if you are in the United States.
To receive calls in Gmail:
Make at least one outbound call from Gmail.
In Google Voice, navigate to Voice settings under the Settings link.
In the Phones tab, click the check box next to Google Chat.