I have the same problem. My phone works only when connected directly to modem. Otherwise, if I'm using a different phone jack I can make only outgoing calls. All incoming calls are going directly to voicemail, and actially I can't see missimg calls, or get the voicemails that people are leaving.The phone doesn't make any dial tones or either recording. It's so frustrating.
"My phone works only when connected directly to modem. "
Well, that's a dead giveaway that something is either wrong with your home wiring or you have a device connected somewhere on your home wiring that you forgot about.
I had an issue with incoming calls NOT going to voicemail as they should have after 4 rings. Turned out that I had forgotten about a multi-function printer (with fax) that was answering the phone line before the Comcast system could pick up and send the call to voicemail. After turning off the auto-answer function in the printer, everything worked properly.
Anyway, if your system works correctly with the phone connected directly to the modem but has issues when connected to a phone jack, you know the problem is inside your home wiring somewhere.