The ADSL micro-filter is a small box that connects to your phone socket. Your phone and router then connects to the filter.
It looks like this:
Finding the filter
It maybe that you don't actually need a filter in the first place: you only need one if your main phone socket looks like one of these (that is, a faceplate with a single socket):
You don't need to use ADSL micro-filters if your main phone socket looks like one of these (a faceplate with two separate sockets):
If you need a filter and can't easily see it (maybe you're in a cluttered office), the easiest way to find it is to follow the cable out of your router towards the wall.
Changing the filter
Gently unplug the filter by pushing the small clip on the side of the plug and pulling it out of the phone socket.
How do I get more ADSL filters?
You can order more ADSL filters at BT Business Direct and most computer shops.
You can still install broadband even if all sockets don't yet have an ADSL micro-filter. But you must unplug any device using a socket without an ADSL micro-filter. Once you've an ADSL micro-filter for it you can reconnect the device.