Land lines do not work after installing new Xfinity modem

... We have no working phones in the house, except for one phone plugged into the back of the Xfinity modem. ...

If you just replaced your Xfinity gateway you should be able to run a standard modular base cord from the jack on the gateway device to the nearest telephone jack to energize your home's phone outlets. That should allow any phone plugged in to any jack in the house to work. If you have a phone in the room where the gateway is located and only 1 phone jack there, you'll also need a phone wiring "Y" connector.

If you just switched your phone service to Comcast, you'll also need to make sure that the previous providers line has been disconnected.

Please be aware that there are 2 kinds of responses in this Forum: Replies and Comments. When you Comment on a post by scrolling down to "Comment on this post here...", I am notified of your response. But if you select Reply, I am NOT notified and may not be aware of your response.

... we already did that and had Xfinity service technicians here as well ...

If a phone plugged directly into the gateway works ("... except for one phone plugged into the back of the Xfinity modem ..."), then there is a problem with the telephone wiring in your home. Comcast's premise techs are not always knowledgeable about telephone wiring, so you might need to call on someone who is.

Please be aware that there are 2 kinds of responses in this Forum: Replies and Comments. When you Comment on a post by scrolling down to "Comment on this post here...", I am notified of your response. But if you select Reply, I am NOT notified and may not be aware of your response.

I just fixed a similar situation. Without realizing, I was using a 2 wire tel cable from the modem to the wall jack. I replaced it with a 4 wire tel cable and the whole house works now.