phone ring OK in the socket ....
phone no ring in the filter
but you've tried 3 different ones ....
so it's either the socket .... but phone ring OK
so that only leaves the wiring
(you don't mention other sockets?)
the line tests OK .... all this means is that the master socket's clear ....
do you have one of these sockets?
http://www.bttorj45.com/BTsocketmasterNTE5.htm l
if you do - unscrew the front plate
inside is a socket that the front plate plugs into
plug your filter in .... see if it rings