On Wed, 18 May 2011 21:12:56 +0100, The Natural Philosopher wrote:

The 3rd wire is are reconstituted by a 'master' socket with a resistor
and a capacitor.

IIRC the 4th wire is 'anti-tinkle and tells local phone extensions not
to ring because someone is dialling out on a LD phone.

No the wire at the junction of the C & R is the "anti-tinkle" as well
as the "bell" wire. It is shorted to the "A" wire (pins 3 & 5) when
the dial is moved off the backstop.

The other one would be earth for earth recall. Very rarely used these
days.

--
Cheers
Dave.