My Speedtouch modem is connected by an extension cable to the main BT
socket. Whenever the phone rings it disconnects the ADSL connection for a
few seconds,reconnects and then disconnects again when you finish the
phonecall.
Also:
If you put a filter at the modem end of the cable (or no filters anywhere in
the connection) then the modem detects the ADSL connection but the above
problem still occurs. However if you put a filter at the socket end (where I
believe it should be) then the modem will not even detect the ADSL
connection, even if the only thing connected is the modem.
Any ideas?

Seventhson wrote:
You have to sit down & think very carefully about where you are
putting your filters, simple as that. All phones need to be filtered
& (here there will be some dialogue with others who believe otherwise)
you need to supply the modem feed via only one filter & only from the
RJ11 port