Melware.org Wiki 
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.
Side A |
Side B |
1 |
1 |
2 |
2 |
3 |
4 |
4 |
3 |
5 |
6 |
6 |
5 |
7 |
7 |
8 |
8 |
So 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)