it can run from anywhere on the line. from an extension socket connected to an extension cable, wired to another extension socket. As long as wherever there is a phone it is filtered. If you have a huge extension then "they" say it can cause issues with disconnecting etc, but i've had no issues in my experience.
Most people don't have a problem but if you do get problems just unplug the phone(s) from the all the sockets before your computer. A pain in the rear end I know, but it worked for a work colleague of mine who's connection kept slowing and/or dropping out. Or do what my dad has done, and wire the phone extensions so that they come out of the main box as well.
unless you have a phone that can cause interferance then using an extension is fine (and you have a filter for this aswell),
thats how i remember the wiring
white blue - 5
blue white - 2
orange - white - 3
extensions have to have orange and white but you will see from the main bt entry they only come in with 2 cables into 5 and 2 and be careful bt cable carrys voltage i think its 80 when using the phone and 50 when dormant ?
well anyway dont strip the wire with your teeth lol