Connecting the Cisco 7281 IP Phone to Xfinity gateway router? Setup steps for phone?

Cisco IP phones only work with a PBX system.  You need to configure one of those first.  I suggest RasPBX on a raspberry pi.  You will need a SIP trunk from comcast business to use.

OK.  So do that.  Setup a SIP trunk.  Im not willing to pay more for phone service.

I expect Comcast to provide IP phone service to residential customers.  I am not a business.

I am workiing from home.  My employer is not paying for my Internet service.

Analog phones are old technology.  I expect everyone to be usiing IP phones within the next year.

Do I need to provide the MAC addressof teh IP phone?

Do I need to provide the MAC address or public IP address of my gateway?

Is there any changes that need to be made to the internal firewall to ensure that phone service transfers to 1 of the 4 Ethernet ports on the back of the gateway?

Rather than telling me that it does not work that way, why dont you tell me how it does work?

I expect Comcast to provide IP phone service to its residential customers, and not through an ATA.

Wow.  First off, comcast does provide IP voice service, the home phone service is transported over SIP protocol to the ATA device that registers with comcast's phone server.  This is all done over IP and not analog lines.  A residental connection will not provide you with the SIP trunking credentials.  For residental connections, you just need a single phone line.  You are working from home, but you still are using it for business purposes.  There is a reason business services are a thing, for business you require a whole other set of service.

Second off, you seam to have absolutely zero understanding of how IP telephony works, so before you come after people who are simply trying to help you, maybe take a moment to humble yourself.

I myself once didn't understand how this all worked so despite your rude remarks, I am going to explain it to you, so that others with this same question will be able to do this for themselves. 

When dealing with cisco phones, there are two product lines, the SPA and the CCUM.  The SPA phones, for example the SPA525g are like an ATA and will allow you to use a remote PBX to peer with and use.  The CCUM line phones like the 7281 you have, require an onsite or atlease in network PBX.  The way you want it to work where you just plug it in to an ethernet port and it works just technically will not work.  It's not how the technology and the protocol was designed, and its not how cisco makes their phones. 

For your 7281 to work, you'll need a sip trunk, a pbx, cisco firmware and config files,  and an internet connection.

You could get a SIP trunk from les net for about $10 a month.  You can order a raspberry pi for about $35, and you can use the RasPBX image that available for free and the OSS End Point Mananger thats also included free with RasPBX to get all of this setup.  This is about as basic as a PBX setup can get, and im sure that plenty of people with knowlage of SIP and PBX would be willing to assist you as setup would take me at most 30 minutes to get setup for you, but you needed to be rude in the begining.

So...  this might be a good place to ask...

I have a "landline" as part of my home comcast Triple Play.  I am going thru a remodel and bought a Grandstream 3380 for my "smart" personal home phone - because; a) I wanted to not only use my phone at a location without a rj11 jack - but b) also wanted to have this phone as an andriod tablet that could be used for video doorbell, and other smart home features.

Problem is/was i needed to register the IP line on the phone...  I called comcast - but they woud not give me the SIP accout information...  now, i know that my landline is voip - so why not allow me register this phone??

Waaay back in 2008, Tom Keating wrote an article "Cool Phones for FiOS, Uverse and other VoIP providers" and in the 12 year since... N.O.T.H.I.N.G.... why?

@jd12122 Do you have any recommendations for cost effective SIP trunking? I'm using Sangoma SIP Station, but it's kind of overpriced. 

I agree with the o/p that it is annoying that Comcast doesn't provide SIP credentials to the residential customer, even though they are basically implementing a SIP trunk with an ATA for their home phone serviece, but alas. I am also working from home and worked around this by installing FreePBX (FreePBX | Open source, web-based, IP PBX management tool) on a spare computer and purchasing a SIP trunk from Sangoma's SIP Station service. I have Yealink and Grandstream SIP phones running nicely with this setup.