It's really VERY simple, and will be explained in the installation instructions! - you need a filter on EVERY item in the house connected to the telephone - or at least every item has to pass THROUGH a filter! (which is effectively the same thing).

While you had the front cover off the master box, and plugged your filter directly into the master socket underneath, it all worked? - all you needed to do was plug the front of the master socket into the phone socket on that same filter. That means ALL sockets in the house are fed through that single filter, and everything works pefectly.

You then just have to find a way of doing it neatly!, as the test solution I suggested above is rather unsightly!.