i have just replaced my master faceplate with a filtered version and run a new extension with both filtered (for a phone) and unfiltered (for adsl) through different twisted pairs in the same cat5 cable to one special secondary box with 2 outlets on one faceplate an RJ45 module for the adsl and a normal phone plug module for the phone by the computer.this is about 5 mtrs from the master box
its a very tidy way to do it and my snr is now exactly the same as it was from the test socket in the bt master box. (with all the wiring hard wired in there is less chance of bad connections)
if you want to see the way its done and where to get the bits
http://www.clarity.it/telecoms/adsl_articles.htm
very usefull info that helped me no end.
they also explained about the bell wire on terminal 3 that most modern phone equipment doesnt need these days that can act as an ariel and so be a source of noise.
(i left mine disconnected on the extension with no probs to the phone )
the trickiest bit was shortening the RJ45 patch cable to about 12 inches
i'm not sure you are supposed to take those plugs apart and reconnect but i managed it eventually as i didnt want any extra cable hanging around near the router picking up signals from all the other equipment nearby.
try to make sure your cable routes are away from things like transformers and mains cables as much as possible as well
my snr is still variable(worse at night ) but i now know its all down to my overhead phone lines outside probably picking up AM radio interferance and not my dodgy cheapo phone extension cables with adapters and filters hanging out by the phones etc that get kicked and sometimes come out.all gone now
i hope this helps someone else as it took me a while to realise it was possible to do this and then find out the right way to do it.
(dont forget to ask for the new longer screws if your original faceplate has machine screws holding it on on older BT master boxes)
current stats for my line
SNR Margin 14 / 12 dB (approx 2db better than before)
Line Attenuation 28 / 12 dB
Data Rate 8157 / 618 kbps