Xfinity Home Voice Setup w/third party equipment

I tried Googling this but didn't get any hits.  It seems Xfinity provides VoIP service for telephone service but when you order the service it doesn't give you a choice for additional hardware nor can I find I find any information on settinng up my own VoIP phone to work with the service. 

On top of all this I had my sister call them since it her service and they told held that she needed a Dual Band Router/Modem which which already has and it activated with no issues whatsoever.  Obviously the people in customer service have no clue.  I told her to just purchase Internet which for the most part works fine and Xfinity customer service seems to be able to troubleshoot. I should give a shout out here to the Gig service which apparently has a higher level of cusotmer support agents and they are people that understand your issues and help troubleshoot as expected. 

Thanks