Have swapped out all the modem-to-wall socket cables the ‘telephony’ front plates on lines 2, 3, 4 for new ones. Line 1 has an NTE5a and an Andrews and Arnold straight-through front faceplate. Disappointment: there was a ‘hollow’ failure on 2020-12-25 and later on I fixed it, by some miraculous intervention, when I swapped modems’ network connections, the problem went away. So the burned out faceplate or cable theory seems discredited as a only new kit was in use.