All I can say is what I gave in my suggestions to jediknight's question. In my case, although I could run through through a normal extension lead direct to the master socket, once I connected via the microfilter I got a very poor response. Plugging the house phone in as well and it didn't work at all. As I indicated the extension cable is rather long. Replacing it with a made up 17 metre cable and everything is OK.
Hopefully, other people will chip in and elabourate with better technical knowhow.