The audio conference engine in Redfire, Candy and Jappix plugins (Openlink) will allow hundreds of people voice chatting in a single room if your server can handle the mixing load. It is completely different from audio/video chat using red5.
With the red5 audio/video, every room occupant receieves a copy of every other occupant's audio/video stream. The limitation on bandwidth at the client end (broadband) and the red5 server limits the number of people involved. In practice, you will not get a good user experience with 12 people. Even Skype has the same limitations.
With the audio conference bridge, all audio in a room is mixing before sending a copy to each room occupant. That removes the bandwith limitations, but imposes more work on the server.
I am using an extension to XMPP called Openlink. Look athe Manage Voice Bridge command
If I have a voice chat underway in a chatroom and another user tries to join in that voice chat it will silently fail
No, What I meant is that the voice bridge is linked to conference rooms. It will not allow you to join a voice conference without being an an occupant of an MUC with the same name. It is a securtiy feature to make sure MUC permissions are also enforced with audio conferences.
As part of this security, you can join an audio conference from an external telephone. If the associated MUC room is pin protected, then you are promted to enter DTMF tones to authenticate.