POTS wiring question

Apologies if this is the wrong forum, but could find no other.

I have a BT 2200 deskphone which only works with the provided cable. My old cable does not work. I see the 'RJ11/?61?' plug on it has 4 connectors while the original only two.

Not a problem EXCEPT I have a line 'hiss' which is not present on my original deskphone using the 2 pin plug.

I can find no information on why the 4pins are necessary and wonder if there is something I need to do to my extension wiring to eliminate the hiss?

Looking more closely, I see the 'new' 4 pin actually still only uses the 2 wires, wired to the outer pins whereas the 'old' 2 pin connects to the middle 2.

I always presumed that the additional contacts were there for the original ringing circuit (well one at least) It shouldn�t be causing an issue.

Does this line have DSL on it, and if so, have you tried swapping out the filter ?

It is fed from a filtered faceplate. Does the fact that that there is no noise on the line on my original phone point simply at the new phone audio circuit? I have not used filters for some years now since I fitted the filtered faceplate. Could this phone need one?

If the 'outer pair' (the only connected wires) were for ringing, what carries the actual voice signal? I cannot find any explanation from BT why this model (2200) needs this different plug.

When it comes to equipment there is no standard wiring for RJ11 plugs.
Some pieces of equipment use the central two connectors 3 and 4 others use pins 2 and 5.

The BT plug uses pins 2 and 5

For further info see Wikipedia.