Please see https://www.xfinity.com/support/articles/troubleshooting-no-dial-tone and https://www.xfinity.com/support/articles/power-cycling-your-modem. If still not working, try plugging a corded phone directly into the modem or gateway. If that works, the problem is in your home phones or wiring. If it doesn't, something's wrong with your Comcast service.
If you can't get it working contact them at the phone number on your bill or 1-800-Comcast, or chat with them at https://support.xfinity.com/chat. If they can't fix the problem remotely insist that they send a tech out to identify the cause and correct it.
If the tech finds bad coax, splitters, amplifiers, or connections in your home (even if Comcast originally supplied them) you'll probably have to pay for the visit unless you have or sign up for their Service Protection Plan (https://www.xfinity.com/support/articles/service-protection-plan, about $6/mo) and keep it for at least 60 days. If the trouble is due to a faulty Comcast phone modem or anything outside your home, you shouldn't be charged.
Comcast may or may not respond to your message here. If they do respond, it may take them several days.
THANK YOU BRUCE W. I GOT YOUR HELP AFTER EVERYTHING WAS FIXED, BUT IF IT WAS NOT I THINK YOUR INFO WOULD HAVE HELPED ME FIX IT.,I ACTUALLY DID WHAT YOU SUGGESTED LIKE INSIST ON A TECH TO COME TO MY HOME WITHOUT CHARGE. WHICH THEY DID. THEN YOU SAID TO PLUG A CORDED PHONE TO MY MODEM, WHICH THE TECH DID. I HAD NO IDEA THE PHONE COULD WORK WITHOUT BEING PLUGGED INTO A WALL JACK, I DID NOT GET YOUR ADVICE IN MY EMAIL ACCOUNT. THIS IS THE FIRST TIME I HAVE SEEN IT. OH I JUST READ NOT TO INCLUDE MY EMAIL ADDRESS . I SEE THAT IS A GOOD SAFETY FEATURE. SLOW BUT SURE I AM GETTING TO UNDERSTAND HOW THIS FORUM WORKS. MY SON IN LAW LEARNED A LOT FROM A FORUM SO I WILL BE A NEW SOLVER. AND YES I DO HAVE FAULTY WIRING LIKE NO GROUNDED PLUGS, WHICH I AM GOING TO REPLACE SOON. AND SHORTLY AFTER EVERYTHING WAS UP AND WORKING MY COMPUTER DID NOT WORK AND I THOUGHT I WAS GOING TO HAVE TO CALL A TECH AGAIN. BUT I THOUGHT IT WAS STRANGE THAT EVERYTHING ELSE WORKED LIKE MY PRINTER, THE MODEM AND MY PHONE. SO IT HAD TO BE THE STRIP OF OUTLETS I HAD THE COMPUTER PLUGGED INTO. MOST OF THEM HAVE A SWITCH TO TURN IT BACK ON WHEN THERE IS AN OVERLOAD ON YOUR OUTLETS BUT THE ONE I USE DIDN'T HAVE ONE SO I DIDN'T KNOW HOW TO TURN IT BACK ON. I LOOKED ALL OVER FOR SOMETHING TO PUSH BUT NO LUCK. DO I TURNED IT OFF AS I DID BEFORE AND THE SECONG TIME IT WORKED AND I'M OFF AND RUNNING AGAIN. I DON'T KNOW WHY IT DIDN'T WORK THE FIRST TIME I TURNED IT OFF AND ON AGAIN SO I AM DEFINETLY GOING TO STOP USING ANY OTHER APPLIANCES WHEN I HAVE IT TURNED ON. OF COURSE THE MAIN THING IS TO GET MY WIRING UPGRADED. THANKS AGAIN FOR TAKING THE TIME TO TEACH US OLD FOLKS.