Two Line AT&T - 2nd line No Dial Tone

I have a two line AT&T speakerphone model ML 17928. I have activated both my home phone and the 2nd line. Until a few weeks ago all worked well. Suddenly I get no dial tone on the 2nd line, but the 1st line works fine. I have Comcast VoIP. Nobody seems to nail the solution down at COMCAST HELP. I restarted, FOLLOW REPS INSTRUCTIONS, even had a rep visit house but he could not figure it out either. Help?