Is Comcast your "wired" service or a Internet phone service. If it is a wired service, you should have a box outside where the line connects to your house. I have Verizon. When there is a problem with phone service, I have to take a "wired" phone outside, open that box, connect the phone in the jack there. If I get a dial tone there, the problem is in my house.
Disconnet all your phones & then connect just one at a time to see if 1 phone has a problem.
The way my house was wired inside is called piggy back. If just one connection is bad, I lose all of the phones. I learned this when the painter sprayed water into an outlet when removing wallpaper. I did not know until he left that I had no phone service. I replaced that jack & got my service back without having to pay Verizon.