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The community's VoIP FAQ contains this post here:
Oct 25, 2011 Question: What is the maximum amount of participants in a conference call?
Resolution: Please check => here <=
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After some digging on my own on this awhile back I found that if your only using JUST the Polycom's integrated Conferencing then the only supported method is 3-way (You and two others) otherwise you'll need to join a 3rd party conference bridge of some kind. We have a Mitel 3300 MCD that our Polycom SoundStation IP 6000 is on and Mitel has their conference bridge to do conferencing on. No doubt there are others though.