I guess I should have mentioned an exception. When the GPO monopoly on telephone service ended and BT allowed individuals to carry out their own wiring, it was under the instructions that this could only be done where BT had already installed the new-style jack. People weren't supposed to tamper with the old style 1/4-inch jacks or hard-wired units.
One good reason for this was that at that time there were still quite a lot of party lines in use. Inadvertant reversal of connections could have resulted in outgoing calls being charged to the other party!
