I have been following this thread but fear I don't know enough about sip and all that to make a helpful response. As reference to the below linux section shows, my relationship with yate is strong though occasionally unsatisfactory. Lately it has been working well, for the most part (no buzz), but I still occasionally get a voice-quality complaint and I'm not sure why the buzz went away or whether it might come back.. As I learn more about yate and how to use it, voice quality complaints are getting less common. I particularly love the address function on yate and its convenience. Have not tried the video feature and not real sure yate even has one.
BTW, I did get a new office phone system but didn't go voip as I still haven't worked out enough of the voip bugs. As many folks use our phone system, I decided that reliability trumped expense. But I still use yate whenever I can for any sort of long-distance calls and will use it more often when I figure out how to use the conference feature.