[ThePillenwerfer]

There'll only be two wires, ie one pair, coming into your house. Internal wiring may use four-core cable but one is only used for ringing and one isn't used at all.

The only thing I can think of in your situation is that the first socket and filter you tried have a higher resistance than the Master Socket meaning there isn't enough current available for the 'phone's "extras." If there was no DC it wouldn't work at all.

You say "Thus there is no line voltage to the 'phone when it is plugged into that ADSL filter." Have you verified that with a voltmeter? There should be about 50V there if no telephones are off-hook and if you lift one it will drop to about 10V.