Telephone adapter

Unless you move frequently and end up in non Comcast areas or, switch providers to get a better deal, you might be better off just buying an Xfinity capable DOCSIS modem with voice.
There are not a lot of voice models just for Xfinity. Some come with Wi-Fi and some do not. I prefer my own Wi-Fi router as it is more secure and I can update or adapt it easier and, Comcast is not all up in it too.
I use a ARRIS Surfboard (32x8) Docsis 3.0 Cable Modem for Xfinity Internet & Voice (Model SBV3202). It seemed the best compromise considering where DOCSIS is at the moment. The only downside is that it is incredibly ugly so, hide it in a closet or the basement.
There are also much faster and more expensive (if you need it, most home users do not) 2 Gbps DOCSIS 3.1 32x8 modems too.
I wish there was an Xfinity voice adaptor. I'd have just bought one of those (if under say, $80) and use it with a perfectly fine, if a little old, Motorola DOCSIS 3.0 modem I already had! For my slowest speed you can get service, it probably would have been fine. But I needed a land line phone…
Third party VOIP options I have tested over the years just never worked for me…
Comcast, Verizon, et al have this all figured out and we are trapped for land line service in monopolies or duopolies in the US… 😞

It's funny. I have spoken to a supervisor inside Comcast and she told me there is such an adapter and it will work. Unfortunately she didn't give me the exact make/model and I didn't get her full name so that I could call her back with additional questions if need be. But others say no, it's not possible.