The program you're likely referring to is
Ventrilo. This has been suggested before, but decided against for several reasons.
First, if we officially ran a Ventrilo server, it would be yet another server that has to be paid for each month. (Speaking of which, it is (actually is, this time)
Donate Your Level Weekend!)
Second, it's harder to feel the immersion when you hear a guy's voice playing a girl, or a 17-year old playing a wizened wizard, etc.
Finally, GMs watch and reward roleplay bonuses based on what they can see in-game, through the client. There's no easy way to do this on Ventrilo, and no way to log it, either.
So while you're welcome to set up your own Ventrilo server, or use
Skype,
GTalk or whatever VoIP program you might choose, bear in mind the above points - and again, it won't be officially supported.