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.