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.
Swapping the filter
We might have sent you a spare filter when you first installed your broadband. If not, don't worry: you can buy one from BT Business Direct.