I have three boxes, all on SIP, and all of them have poor voice quality when on a conference bridge and too many people are on. Various scenarios -- sometimes it's all VOIP phones calling in, sometimes it's cell phones and local callers to the box calling into a box on the west coast. Sometimes I get jittery voice/missed packets, last week the cell callers reported a fading in and out. My hardware vendor says to add their analog carrier card. That it will provide a timing source, which is done all in software when the card is not present. Other voice does not seem to be affected.