Checking the ADSL connection is as near to the BT Exchange as possible (eliminating potential ‘interference’ and parallel impedance of extension telephones and cabling).

Picture 1

Picture 2

Picture 1 is a standard BT Master socket with a microfilter showing modem lead and telephone lead.

Picture 2 shows customer serviceable section..  This is removed (to connect telephone extension cabling) by unscrewing two screws shown and pulling forward.

Picture 3

Picture 4

Picture 3 shows the front plate removed.   The extension connections on the removable plate can be seen going through the access hole in the body to the coiled up end of the extension cable.  On the bottom right of  the main box can be seen the standard telephone socket which is connected behind to the incoming cable by BT and SHOULD NOT BE TOUCHED!! The mating telephone plug is an integral part of the front plate and connects the BT line to the house when the front plate is refitted.

Picture 4 shows the ADSL modem plugged via a modem/telephone plug adaptor (not in this case a microfilter) to the ‘BT’ socket which is the nearest you can get to the exchange and all sources of potential ‘interference’ generated through the house wiring are eliminated because there is no connection between.

The above should be read in conjunction with http://www.wppltd.demon.co.uk/WPP/Wiring/UK_telephone/uk_telephone.html on extension wiring

Here is a further illustration:

Just to reiterate arleyb’s instructions – the basic idea is to connect your modem/router to the concealed socket (B) which should provide a better quality signal than the normal BT Master socket (A).