This took a whole bunch of experimentation. But I think we're in the ballpark now. This is our telephone.
It's an inexpensive handset hooked up to an iPod Touch. The Touch is hooked up to our Mac Mini (which serves mostly to just power the iPod.
We're running Skype. This way the telephone can be answered from wherever. In fact, you can pick up the handset and the iPod and walk to the conference area or anywhere you like.
Right now the only downside is that the ring is very quiet. But if we have enough computers also set up with Skype we'll have no trouble hearing them ring. The Mini is serving both as power and as a ringer.
The Skype account is unlimited minutes to US and Canada. So go crazy with it.