Sorry, don't have Comcast's phone service, but I do use VoIP on Comcast Broadband. My VoIP provider is VoicePulse, and my monitoring company is NextAlarm. I had some issues using NextAlarm's "preferred" protocol, Contact ID. I had to switch to SIA signalling to get reliable reporting.
I don't know specifically what would make Comcast's VoIP different from other providers. I do know they use a separate "band" for VoIP... it is not part of your internet bandwidth, and supposedly doesn't run the risk of getting choked off by excessive internet usage. So for one thing, you wouldn't need to worry about using any kind of QoS - that's already built in to Comcast's network. There may also be some latency advantage, since Comcast's POP will be local to you. For me, I believe VoicePulse's network is homed in NJ, so calls I make have to travel via the internet to NJ, then go from there. I know when, for example, I call my cell phone from my home phone to test different QoS settings, with one phone in each ear, there is a VERY noticeable delay between me saying something, and hearing it out the other phone. I don't know how much of that is due to the VoIP network, and how much is due to the cell network, but it's enough to make you wonder.
