We're having the same problem tonight. Call Comcast. We just had service out today and everything was working fine, or so we thought, but realized tonight the phone isn't working. They couldn't fix it from within so we have someone coming out tomorrow. We have a lot of problem with our phone service. I'm honestly sorry I every changed over from a regular phone line.
Double check that the telephone cord is plugged in firmly to the back of the cordless phone base, the wall outlet, the outlet where the telephone modem is and at the L1/L2 jack on the back of the telephone modem. You can also fully reset the telephone modem by pressing the indented reset button on the back panel of the modem with some kind of poker like a toothpick or end of a pencil.
It boils down to the cordless phone base is not getting the dial tone line voltage from the telephone modem to the cordless phone base. There could even be a short or open in the wiring in the home or one of the jacks depending on how the home is wired and how the telephone modem is feeding the inside phone wiring in the home.
We went through all of this today and spent over an hour on the phone with customer service. Personally, I think they may have an outage that they aren't admitting to, but we'll find out tomorrow when the repair guy comes. We had phone service today when the repair guy left but now we have nothing. Never should have gotten rid of the old land line.