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Provided that you have voice service on your ADSL line, you can put a filter at the 66 block (or 110 block, or NID, whichever is your DMARC) instead of one at every station (phone) but you need to route unfiltered signal to the DSL modem. The only issue with this is that you need to pull a clean pair from before the filter to the DSL modem if you do not have a good quality unused internal pair.
As an example, at my house I have ADSL Service (really Paradyne MVL, but close enough for our purposes). The pair comes into a NID on the side of my house and from there goes to a 66 block in a closet. I have all my home station wiring pulled to the 66 block, which has a filter inline before it. From the NID, I have one other pair, which of course is unfiltered, all the way to the DSL modem.
Conventionally this separation is done via a "splitter" which gives better results but is more expensive and hard to find through conventional means. A splitter is normally installed at the NID and is a 3 port device. It splits the combined POTS+ADSL signal to POTS and DSL outputs. POTS output will be low pass filtered so high frequency DSL signal will not interefere voice and the DSL output will be high pass filtered (so voice signal will not interefe the DSL modem).
Also read About DSL for lots more information