Are you already a Comcast Xfinity customer that has the Xfinity Voice service perhaps using a leased gateway/router and just purchased a gateway/router and want to use it on your account rather than the leased one? What is the particular Arris gateway/router that you purchased? Just asking so I have a better understanding of your situation so I will know how best to answer your question.
Yes I am a current ComCast customer and have Voice and lease an xfinity Gateway/Router.
Purchased an Arris/Xfinity model TG862G/CT.
Thank you for providing the additional information. Yes you can keep your existing Xfinity Voice home phone number when you switch out your leased modem/gateway to use the one you purchased. In fact I just did that same thing this past weekend. There is an excellent article here in this forum at the location of http://forums.xfinity.com/t5/Home-Phone-Service-Equipment/How-to-purchase-and-install-a-voice-modem/... that details what you need to do. After you scroll down the list of modem/gateway options, you will find the steps you need to do. I followed them and did not have any issues at all.
Wish you the best in your upcoming efforts.