[engineer]

not entirely true, depends when your house was built and what type of internal phone wire was used. I had to run separate cat 5 wiring out to the outside phone junction box where the DSL was added so it is split by the phone company now from the old inside phone wiring and into the new cat 5 wiring. That way there is no interference with the existing phones and I did not need a filter on each phone. Also that gives me a good wire to the DSL modem particularly if you hard wire in a network instead of wireless which I have done. splitter porvided by DSL company along with extra splitter in case the one fails.