In fact it was "terminally intermittent" it came on for ten seconds and then was lost again.
If its live, what you *shouldn't* do next is try and trace miles of cabling which has been there for maybe 20 or more years which goes all over the house behind furniture etc. moving piles of stuff which have accumulated in the interim in the process. And then let despair place its icy hand on your shoulder as you contemplate having to trace the fault somewhere Somehow. Days, weeks, months ?
In fact the next thing you should have done was check the ADSL filter which was plugged into the master socket by plugging the phone into that and see if the phone is still live.
So those piles of spares that have sat in the draw have finally found a use.
It would appear a faulty* ADSL filter on the master socket can knock out everything at a stroke. Including the "unofficial extensions" ran off it maybe 30 years ago
Obvious really in retrospect. Five minutes Googling would probably have sorted it all out.
* again how it developed a fault having sat in the same place totally unmolested for years is another matter.