When using a BRI ISDN card in NT mode (Network Termination), a crossed cable must be used.

This cable also needs (depending on the cards port) 100Ohm resistor termination.

3/4 and 5/6 are the crossed lines.

For the termination, you need to place a 100 Ohm resistor

* between 3 and 6

and one

* between 4 and 5

on both sides.

(It seems to work with termination on one side only too when the cable is short)