Only thing that produces transients or spikes is the phone, thats blocked by the low pass filter. Doesnt matter where the voice goes so nothing filters it out. Adsl can cause distortion on the voice at the phone so the low pass filters that out too.

Clear line from the BT exchange to the Modem.

I understood where you was coming from Serious with that before you scribbled that picture out.

there is some basic surge protection equipent in addition to the low pass filter which is what everyone else may be thinking about, but its just that, surge protection and not filtering for adsl.

cant remember the exact name of it but its basically a gas resistor of sorts that spits anything above x volts back down the line.

its been present in normal bt style master sockets since as long as theyve done them, the filter also contains the components to transform the copper pair into the copper pair + bell wire ala master socket style.

just a little off topic, there are "secondary" extention sockets which are missing all these components and require the 3 wires going back to the master socket which generates the bell wire for it,
you can see the difference easily as theyll be missing the capacitor and resitors from the circuit board.

see http://www.wppltd.demon.co.uk/WPP/Wiring/UK_telephone/uk_telephone.html for more info on master socket and secondaries.