Sounds like you have Call Forwarding active.
and go to Call Forwarding on the left side menu. You will then have the option to change the number of rings on your phone before it's forwarded or goes to voicemail, or otherwise modify any call forwarding settings.
bilburnet -- Were you able to get this fixed with the information darkangelic provided?
I did fix my problem with no help from anybody on the internet including Comcast.
Turns out that there was some corrosion on the phone wall jack where the wires connected.
I cut and cleaned the wires and replaced the phone jack and it has worked fine ever since.
I am not sure what the line voltage should be but if you can have someone check it with a meter
(all of your phones and connections). Evidently I was getting enough to ring the phone initially but
only one time. Not sure why this is but that was the symptom and the fix for my issue.
I am curious to know if this is your issue or something else?
I had the same issue on my phone. It turns out this is a side-effect of the NOMOROBO service that Comcast has connected via a 3rd party partner. If you have this enabled (you can see this from your Xfinity account under phone>Settings>Call Forwarding), I was told by tier 2 support that the phone will ring once - allowing the NOMOROBO to look up the caller - then immediately drop the call. NOTE: No caller ID is even visible during the one ring.