You just need the red and green wires ..

"There appear to be 6
wires coming into the box - all with solid brown insulation"
Which ones of those are red and green? Bob

Red and green are the standard, but someone hooking up a phone is
not compelled to use the standard colors. They would have saved you
some trouble if they had, but they had this 6-brown wire on hand,
and picked two wires and forced it to work. You might be able to
tell by looking at the ends of the wires. Maybe only two of the six
have been "stripped" (had half an inch of insulation removed), or
are otherwise different from the four unused wires, from having been
used in the phone circuit.
Then you have to test which of the two wires is positive and which
is negative, but if you just hook up a phone to them it might work
either way.
Really though, you'd be better off replacing the 6-brown with
modern modular-plug wiring.