Was unsuccessful in installing Nomorobo, so followed its instructions on turning it off (simply by removing its phone number).
Unfortunately now, calls disconnect whenever we answer the phone (number of rings does not matter). Outgoing calls are fine.
Comcast is unable to help (and as many here have noted, the call center reps don't seem to know about Nomorobo).
I've reset - disconnected - reconnected - unplugged - turned off and let sit for an hour ... and no luck.