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[Dicebot]

For voice conferences with large amount of participants (20+) 
usually stuff like TeamSpeak, Ventrilo and Mumble is used. It 
requires some special approach - for example, it is possible to 
setup SIP voice conference but because of SIP architecture it 
simply won't scale well for large amount of participants. And 
XMPPP/Jingle support for conferences is still 
experimental/work-in-progress.

For video conferences I am not aware of any free solution with 
better capability than Google Hangouts. It is quite a challenging 
task actually.