I know that, when the old phones used potted modules for the electronics, there were a good number of those that were made to look alike to the original GPO potted blocks, to be installed in the phones servicing certain locations. Identical box and potting (of course, seeing as the builders had the way to get the right potting boxes, potting compound, silkscreen prints and wires in the potting), and often the only way to tell would be an extra wire to connect the other end of the hook switch, nominally there as "contact interference suppressor" connection.
Of course when installed the lines also were monitored...... But this allowed the room to be listened to as well.