[Hampshire] [OT] BT phone lines

Evening all

Does anyone know how ADSL broadband works with phone extensions?

I have two phone points on the same BT line. Trouble is - the extension has been very professionally fitted - there's no indication as to which is which (i.e. which is the master socket and which is the slave). There's no socket coupler fitted to either point and the cable runs under the floorboards - and the wife won't let me investigate!

As such, which (if not both) do I need to fit the microfilter(s) to? And if I only fit a filter to one, will the other be usable for calls?

Hope I've explained that clearly enough - and sorry if this is ADSL for beginners but I'm stumped.

Cheers
Rob