On Sat, 15 Dec 2018 14:43:50 -0000, Harry Bloomfield wrote:

Kristy Ogilvie brought next idea :
Where is this protective ground? There certainly isn't one in any BT master
socket I've seen, just a couple of twisted pairs coming through a plastic
tube to the socket.

Some service lines would be fitted with a ground connection in the
early GPO days, but I doubt any are now. Most likely there will be a
ground at the pole or the cabinet.

I assume this is only for overhead lines anyway, mine are underground.

Strangely some newer houses in the next block have overhead phone wires (but underground mains wires). Did they forget them when building or something?