The best way would be to get a third party voice chat client. The in-game voice chat is horrible quality and people just do no use it. Two of the main ones are Vent and Team Speak.
Voice chat can only be used in online multiplayer matches. You just speak into the microphone to voice chat.
Video chat on Gmail is legal. You can do it via Gmail chat or Gtalk. Hangouts is the latest name of the chatting system.
To do voice chat you have to enable video chat. You can also enable a phone call from your chat window. These are your two current options.
Voice chat costs in Gmail your credits. You can add credits worth money to your account. Video chat can be free of cost.
Nope. Only text chat.
No.
Voice chat was added in patch 2.3 with zul'aman.
Voice chat on your computer happens through passing data over your Internet connection, not standard "voice" phone lines.
On Windows. It should come with msn messenger which has voice chat in it (which is not very useful because then the other person needs msn as well)On Mac: It comes with I-Chat which I think has voice chat on it (but again the other person needs I-Chat too)Short of that, use a phone?
You can voice chat with Google or call with it. However this calling is not free of cost. It costs you charges per second.
Support for Google voice and video chat can be found in the support section of the Google pages. Google offer several help pages to answer many questions about Google voice and video chat.
A new private chat room can be created from the XMB (while not in a game) by first selecting Friends List and there should be an option for Start New Chat. From there, selecting Voice chat will create a new private chat room.