Footballus vita est
Join Date: Jun 2002
Location: Portland, OR
Bikes: Trek 4500, Kona Dawg
If the filter has an ouput for the phone, it's probably intended for a dialup modem (who used dialup when the have DSL, though?). Here's the lowdown:
Your phone operates in the audible range (16 Hz -22 kHz, actually less than that, but that's nitpicking). Dialup modems also use this range (you know the fax machine beeping and squeeling?) because signals in that frequency range can be passed through the phone switching equipment to a service providers modem on the other end, even if it's on the other side of the country. DSL, however, operates at frequencies above what the human ear can hear. It has both a higher frequency and a wider frequency range (aka broadband), which allows it to be much faster than dialup. The catch is that the signal will not be passed on telephone switching equipment, so the service provider has to have equipment between your phone and the switch to process the signal, which is partly responsible for the higher cost.
Now, back at your house, the modem processes the signal for the benefit of your computer. However, even though the DSL signal is to high-pitched for you to hear it's possible for the signal to cause interference on your phone. I believe this is due to an effect called aliasing, where electronic equipment picks up part of a high frequency signal as a low frequency signal. This sounds like really bad static. The filters are a device called a low pass filter that only lets signals below 22 kHz pass through it. That means phone signals will go through, but DSL won't, so it can't be interpreted by the phone as static. If you plug your DSL modem into a filter, you shouldn't be able to get a connection because it cuts off the DSL portion of the signal (although some filters may have enough transition zone that you can still get a crappy connection).
"The internet is a place where absolutely nothing happens. You need to take advantage of that." ~ Strong Bad