You must be talking about ADSL then.
You need a filter for each point to which you have a phone connected. Unused sockets (nothing plugged in) do not require one. I should have said "voiceband appliance" rather than phone in that FAXES & POTS modems (including the 1 in the dreaded $ky digibox) also need filters.
Chris Muriel (1 ADSL line shared with phone, 1 separate ISDN line, 1 separate fax line,all BT + 1 NTL domestic phone line , cable STB & domestic cable broadband - wires everywhere).
Manchester.