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You should place an ADSL Microfilter on every phone you have at the jack, but not the jack the DSL line is plugged into. Most in-line microfilters can be found at your local computer retail store for under $15. If you have a wall-mounted phone (ie: older rotary phones) you must buy a different type of filter than runs for around $30.
Wall mount filter:
Also read About DSL for lots more information