Panasonic KX-TDE100 Installation Manual page 71

For a BRI Card
Attach the ferrite core, then close the case of the ferrite core. Attach the ferrite core as close to the card's
connector as possible. The ferrite core is included with the card.
Note
For every 4 cables, use 1 ferrite core; a ferrite core holds a maximum of 4 cables.
When only 1 or 2 cables are used for connection, wrap the
cable(s) once around the ferrite core.
When 3 or 4 cables are used for connection, simply pass the
cables through the ferrite core.
3.2.9 Attaching a Ferrite Core
Installation Manual 71