I quickly called Comcast and got a person in a reasonable amount of time. They said it looked like several others in the neighborhood were affected, so it was likely an equipment problem or upgrade on their end and things should resolve themselves within 24 hours.
"You mean I'm going to be without my phone line for up to 24 hours?" I asked disbelievingly.
"Yes." said the customer service rep. "I'm putting in a ticket, though. Your signal strength is a bit out of range."
So the technician shows up today during the promised two-hour interval and... power-cycles the modem. Great. If I'd known where the secret bypass-the-battery-backup button was, I could have done that myself. He doesn't appear to have tested the signal strength, and I haven't checked the quality of the TV signal myself, so they may have to make a second trip out.