Getting a neon light on the handset of an old GPO 700 series telephone to illuminate
This is the handset no. 7.

In the ideal arrangement the lamp is powered from the ringing current
on the line and flashes until the call is either answered or
abandoned. Such an arrangement can be achieved if automatic ringing is
derived from a 75-volt ringing generator and the lamp is connected
directly in series with the bell.
As shown in Diagram N 1901 a resistor is connected in parallel with
the lamp to extend its life by limiting the current flow

In those days of course the bell would be 2 x 500 ohm; for modern use there should be a 3k3 resistor in series with it to convert it to high impedance parallel connection.
The Diagram N1901 is available from Sam Hallas' website.