Open your Windows Live Messenger and invite new recipient whose user id is email@example.com. Yes, this same ID is used in Google Talk too.
Type like CALL firstname.lastname@example.org in the chat window to place an outgoing call to Google Talk user email@example.com.
In a second or two you will get an incoming call from firstname.lastname@example.org which you have to accept
When your call is answered on the remote side, "Remote answered. Now talking.." message is displayed. From this moment, you may talk.
Type 'HELP' for more information in the chat window.