If you are using Google Talk and want to connect to a friend who uses yahoo messenger you will have to play around with a very small trick to make sure you can both connect even though you use different instant messenger services.

From your Yahoo Messenger ID, Add this ID - ‘gtalk2voip011@yahoo.com’
You wont get a confirmation that the ID has been added and you will find it offline but still you can start sending messages through it. The first step would be add this ID to your yahoo account.

Once the adding friend process is completed you need to send some message to this ID which will be forwarded to your Gtalk ID first in the form of a Add Friend Request.

In the Gtalk account a confirmation option is shown with the message of ‘Add as a Friend?’, which once you confirm you can start chatting within two different instant messengers directly and also make voice calls.

Other Useful Commands you can use with GtalkVOIP are :
MYPAGE - Display URL to your personal account page.
WEBCALL - Display URL to your personal Web Call.
DICT _word_ - Make a request to DICT.ORG for word definitions.
MSG _to_ _text_ - Send off-line message _text_ to user _to_ .
VMLIST - Display content of your voice mail box.
VMPLAY _id_ - Play voice mail message number _id_ .
VMDEL _id_ - Delete voice mail message number _id_
VMSEND _user_ - Send voice mail to other gtalk _user_ .
VMSET _id_ - Set _id_ as a default welcome message.
CONF - Create conference room for you. Room cookie will be sent back.
JOIN _cookie_ - Join someone’s else conference room using _cookie_ .