This is what I found:
a very long LJU cable and following it, where does it go...
it disappears OUT THE WINDOW (along with some Cat5e).
After going down three floors and round the back of the building it seems to go into another room somewhere further along the building. Unfortunately all the other rooms it could go to were locked so it was not possible to test the handset or investigate the need for this type of cabling.
I will now try to find somebody and explain why it might be better to use a socket in the room or, if they are all in use, put in a new socket.