1. Unplug all telephones and other devices including any filters from your phone sockets and see if your modem/router will work when plugged directly in to the socket.
   
   
2. Try plugging your modem/router directly in to the master socket.
   
   
3. Behind the master socket there is a "test socket". You can undo the 2 screws on the faceplate and access the test socket. Do not however remove the actual socket or expose any of the BT wiring.

*When trying to plug a router or modem directly into a phone socket without a filter, you can use one of the cables that is normally used for plugging in a 56k dial up modem.

If your problem is resolved at stage 1 or 2 then its likely a problem with either your filters or the way any extensions are wired up.
~ Check that you have installed your filters correctly.
~ Add one piece of equipment and filter at a time so that you can track down where the problem lies.
If BT find that your problem is at these stages they will in all likelyhood make a charge for being called out.

If your problem is solved at stage 3 and particulary if you were having problems related to a low SNR margin then consider replacing the faceplate on your master socket with a special NTE5 adsl faceplate.
See my page on low SNR for more details.