Open your Windows Live Messenger and invite new recipient whose user id is firstname.lastname@example.org. Yes, this same ID is used in Google Talk too.
Type like CALL email@example.com in the chat window to place an outgoing call to Google Talk user firstname.lastname@example.org.
In a second or two you will get an incoming call from email@example.com 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.