Sparks lent a hand in testing out the setup (screen capture below). That's an inbound Icecast stream (muted so we could talk), an outbound Icecast stream (for podcasting), two cell phones and two Skype clients. There was a little bit of echo but I think that was cross-over due to the proximity of the handsets.
It's surprising to see that IPKall and FWD allows more than one concurrent inbound call. I'll need to do a bit more research to figure out what the limits are, both remote and on my system (what with all the other daemons that are running).
Note that I've updated the mute/unmute code.
Thanks for the assist, Sparks!