You will usually need a Filter/Splitter or a Central Filter/Splitter installed near the ADSL modem and an In-Line Filter connected between each piece of analogue equipment (telephone, fax machine, alarm equipment etc) and its associated socket.
The important thing to remember is that the Filter/Splitter (and your ADSL modem) needs to be connected directly to the incoming exchange line and must NOT pass through an In-Line Filter first. An ADSL Filter/Splitter connected to the analogue side of an ADSL In-Line Filter WILL NOT WORK! If you have any doubts you will need to trace the telephone cable from the street and determine which is your "master" socket.
Assuming that the ADSL modem is correctly operating and you are unsure where to place In-Line filters, the best thing to do to ensure optimum performance of all equipment is to install and In-Line filter between every item of analogue equipment (telephone, fax machine, alarm equipment etc) and its associated socket.
Use the drawings below to ensure that you understand what frequency spectrums are passed or blocked by different types of filters.
A Central Filter/Splitter.
An In-Line Filter.