" 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.
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