In game voice chat works but separate clients generally work better. Two popular clients are TeamSpeak2 and Ventrilo To use them you need the client and a voice server. At least for now TOG/OG is using the TOG Ventrilo Server as needed for raids.

• TOG/OG post on the TOG Ventrilo Server. You must be a TOG/OG member to view this post.
  

• TOG post on Adjusting Sound on a Ventrilo Server - has information on optomising voice setup.