@grich, apologies, I didn't notice your response from the 23rd until now. No, it's actually a digital phone, not IP.
Something changed with that device. I traced wiring from the wall jack through the closet(s) and re-punched the 66-blocks just to be sure but no change yet. I may replace the wire on my next trip there. If still no success, then I may swap the extension with another address on the card to see if it makes a difference.