I have found;
"Many years ago the UK telephone system adopted a different policy to that in the rest of the world. Here in the UK we made a choice to move a component called ‘the ring capacitor’, which enables the telephone to ring, out of the telephone and into the socket on your wall. It was a good idea at the time because it made it possible to stop the ‘tinkling’ that occurred on other extensions when an old rotary-dial telephone dialled a number.
However, there was a downside: To get the ring signal into the telephone we now
have to run a third wire from the socket to the phone. Also, since we only need one
ring capacitor on our line, and it is located at the 'master' socket, we have to take
three wires to all our extensions, rather than the two wires which come to the house.
On most modern wiring you will find the original phone signal connected on the 'bluewhite' pair of wires and the extra 'ring' signal connected on an orange wire, though this convention is not universal.
Now, since ADSL filters only filter the main telephone pair, they must re-generate the ring signal themselves. All of this is of only passing interest but it does mean that you only need to feed an ADSL filter with two wires. The ring signal for the third pin on the attached phone is generated internally."
I'd like to see "many years ago" quantified. The reference to dial phones suggests the 1960s, i.e 50 years ago. It was certainly my understanding that the ring wire is no longer required but maybe as we move from ADSL to FTTC we need to unlearn the "disconnect the bell wire, it acts as an aerial and introduces noise" mantra?. But if that was true then everyone who has ever disconnected their bell wire, and that must be tens of thousands (?) would be suffering this problem?
I'm doing nowt until I get some more advice!