When you have ADSL (or ADSL2) service that is sharing a line with another telephone product such as a normal telephone, PABX or fax you need an ADSL filter on each and every product connected to that line except the ADSL modem itself.

Just to state that again more clearly, on an ADSL line all devices, phone, faxes etc require a filter EXCEPT the actual ADSL modem.

In order to make this easier where the ADSL modem is sharing a socket there are double adapter products available as shown below:

If you plug in your modem router correctly (as indicated) the ADSL modem router although is physically plugged into the filter it does not go though the internal circuitry and is not "filtered". Below is an example of the physical hookup.

If you don't have a phone or other device (as shown above) sharing the socket with the then you don't need an ADSL filter at all (for this socket).

If you have other hard wired extensions in your house or office each of the other extensions will need an ADSL filter as shown below:

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