Many of you will quite often double up you phone line / fax with your Internet connection. Previously we have used ADSL splitters that plug into the existing standard BT socket. More recently I have been getting a ADSL faceplate to swap out the lower part of the BT NTE5 socket, these are great because they are less likely to get snagged / break and a far neater solution.
The beauty of using one of these faceplates is that you can run extensions from it that already have a filtered connection. Therefore if you connect pins 2 & 5 on the 6 way IDC block (there are some 3 way versions out there, but again the pins are the same) to your extension you will get dial tone without the worries of your internal cabling causing you issues with your ADSL connection.
Its also worth nothing that you can get a unfiltered connection perfect for your ADSL modem by connecting pins A & B. This is good if you would like to put your router in a different location to your master socket and still get the benefits of a central filter.
You could make it a nice neat solution by terminating the unfiltered connection on a RJ11 socket. Also that way it will deter someone from putting in standard telephone and causing disruption to your Internet connection.
We quite often use a CAT5 cable to get the point where we need it (for example where the FAX is located), a nice neat trick is to cut the end off a RJ45 cable and crimp the blue and blue/white pair into pins 2 & 5, this should be the middle pair on a properly made cable. You can then plug a RJ11 directly into the CAT5 socket.