In reply to:
Faceplate filters won't do anything for extensions used for ADSL, because these extensions are not filtered.
To clarify that, an extension into which you plug an ADSL device must not be filtered. That means it must either be plugged into the ADSL socket on the front of the face plate rather than the normal connections on the back (which are filtered).
A better solution would be to obtain a faceplate such as from ADSLNation which has A and B sockets They provide an unfiltered circuit similar to that coming in to the back of the master socket. At the other end, if you do not need a telephone, you could just fit an RJ11 socket (pins 3 & 4); if you do need a telephone there, one of the following:
1. Buy an NTE5 socket and install it as a "secondary master" and then fit a filtered face-plate.
2. Buy a simple filtered face plate.
3. Buy an ordinary telephone socket and use a filter.