I immediately clicked the icon and have a similar chat window at the bottom part of the mail, similar to Google Chat.
The feature requires you to download a plugin enable to start using it.
Google Voice is not just for PC-to-PC call, in fact it is for international landlines and mobile phones.
Watch the video below to know more.
Checkout rates here. To know more about Google Voice visit the official site http://www.google.com/googlevoice
Share and Enjoy!