An ADSL filter, or micro-filter, is a small electronic component that fits between your phone line and a regular voice device, such as a phone, a fax, or any device with a regular modem such as a cable box, alarm system or digital TV. When DSL (ADSL) is provided over voice lines, all devices in the house except the DSL modem must be connected through filters. The filter protects the devices from high frequency noise. They are low-pass filters.
The filters supplied with your Bonded ADSL router kit are used to channel the DSL signal directly towards the Bonded router. Without the filters in place, your ADSL signal would travel and interact with every telephone device in your house before getting to the DSL modem. This would result in either a loss of synchronization with the DSL signal or very poor performance. Make sure you have all phone lines with devices attached filtered.
You can plug your modem into micro-filters (ADSL side because otherwise you won’t get sync), not in-line filters. On a wall mount filter, the DSL goes in “Line” if it has two. Examples of the two:
Above : Standard Pluggable ADSL Filter
If you need microfilters – you can purchase them here http://clickandkeyboard.com/product_info.php?products_id=10429