Xfinity Voice but lacking Telephone Port

I have xfinity internet 1.2GB service and have my own gateway/modem/router (single unit: Arris SBG8300). I'm trying to add xfinity voice but my gateway doesn't have a phone port. Looking for a way to add a phone port without replacing my current gateway/router/modem. One option is to put another modem (with a phone port) in parallel with my current one (but not sure if Comcast allows 2 modems without a large extra charge). Another option is to put a device behind my current gateway (such as a Grandstream GS-HT802 ethernet to telephone adapter or simply a secondary router with a phone port) to give me a phone port. But I can't seem to figure out how I'd configure the device for xfinity voice (SIP server settings, etc.) All of the setup instructions seem to assume I'm using a combined gateway/modem/router with telco port provided by Xfinity. Any help is appreciated.