I have a land line ( 2 actually) and the quality of the call seem to have degraded recently. My phone is plugged directly into the modem. I bought a new phone thinking the old one had an issue, although both seem to be about the same poor reception. what can be done to improve the reception . I would think this should be as good as skype which is crystal clear.
A few thoughts come to mind that might help improve the quality of your land line service as they have helped me out in the past with my Xfinity Voice service where I do have it set up like you with the cordless base unit plugged directly into the back of the cable modem/gateway unit.
I have experienced that if the base unit of the cordless phone is too close to the cable modem/gateway that there will be some interferance going on. I think it is due in part to the signals the base unit of the cordless phone sends out to communicate with the handsets throughout the house and that has caused some issues on the phone line.
Verify that the Comcast cable going into the back of the cable modem/gateway is finger tight secure. I had one instance where I found this wasn't the case and after tightening it up everything was back to normal. I suppose over time with the occassional moving around to dust and clean it had just worked loose.
Also might try verifying that the phone cable between the cable modem/gateway and phone base unit is plugged in securely in both ends and it might even be the age of the cable so a new one might help out.
Hope these suggestions help you troubleshoot your issue.