As I see the world, the purpose of the filter is to prevent DSL tones from disrupting telephone conversations.
They also prevent the telephone from absorbing too much of the higher frequency DSL signals.
DSL is a radio wave (not tones) that travels on twisted pair wires. Anything between a home and CO subverts that radio wave. Nothing - especially not filters - must be in that connection.
POTS devices - ie a telephone - will 'eat' DSL signals. So a filter is installed between that DSL signal and each telephone appliance. Filter blocks DSL signals so that the appliance cannot 'eat' DSL signal strength.
Surges are a completely different anomaly. Surges seek earth ground. If that energy is permitted inside any building, then the surge will increase voltage as necessary to blow through - to get to earth. Surge protection has been well understood for over 100 years. If energy is connected to earth before the wire enters a building, then no surge damage. It is that simple.
Does BT disconnect phone service before every thunderstorm? Of course not. BT’s electronics are threatened by at least 100 surges with each storm. How often is your town without phone service for four days while they replace that computer?
BT does what every informed consumer does. Every wire inside every cable connects to earth ground before entering the building. Cable TV does this less than 3 meter connection with only a wire – no protector required. Telephone and AC electric will not work if earthed directly. So a protector connects each wire to earth.
That is what a protector does. It does not magically make surge energy disappear as so many will claim. It connects massive energy harmlessly to earth. Or it makes appliance damage easier. Your choice.
BT wants protectors up to 50 meters separate from electronics. That separation is critical to making better protection. In your case, the protector is located at the master socket. And that protector must connect ‘less than 3 meter’ to the same earthing electrode used by AC mains. Yes, every wire must connect ‘less than 3 meters’ to single point earth ground. These concepts have been used for over 100 years so that direct lightning strikes do not even harm the protector.
No filter stops surges. DSL filters are for keeping DSL signals away from all other phone appliances. No filter stops surges. Surges are energy that will block through any filter (ie the destroyed DSL filter). Surge protection is always about where that energy dissipates. Either you give energy a short path to earth – harmlessly outside the building. Or that energy will hunt for earth destructively inside. Filters and surge protectors are two completely different and well understood solutions. Routine is to use DSL computers during every thunderstorm without damage.
