I think
http://www.clarity.it sells such things. Have you got a cheap
filter on the Sky box? You might want to try swapping around the ADSL
filters and see if that helps. Also, do you still get the problem with the
extension but nothing connected to it i.e. disconnect Sky box?
BTW, if you have 6 core cable, you might consider Clarity's consumer unit.
I have a single faceplate with phone and ADSL outlets wired, via 6-core, to
the consumer unit (modified box with 2,3,4,5,A,B terminals. All my phone
extensions are now isolated from the ADSL by the filter in the consumer box.
I've not had any problems, despite being able to hear Capital Radio on the
phones!
BTW, I don't work for or have any connection to Clarity.
Paul DS