recommend a open protocol open source voice phone call client?

[Jeremy C. Reed]

Looking for an "open" protocol, open source voice phone client that works 
without using Linux emulation -- and can work with more than two clients 
in a conference call situation.

And having Windows and MacOS X clients is fine too. But client is not 
needed as long as the protocol is standard for existing clients there too.

(I do have access to a Asterisk server, but I want to attempt this without 
using a server. If that isn't doable, please let me know and I will then 
use the Asterisk server.)

The goal is to get rid of proprietary Skype for BSD-focused conference 
calls.

I don't know SIP or H.323 -- do these require a server or can multiple 
clients connect to each other for a working conference phone call?

I don't need connections to real phone network.

What do you use? Any recommendations?

In ports, I see: cphone, kphone, linphone, ohphone, ekiga, kiax.

I also read about PhoneGaim, tkphone, KCall, SFLPhone, wengophone, 
and Twinkle. So many choices, need to know where to begin.

Any pointers to URLs on experiences on FreeBSD would be great.

Thanks!