On Tue, Dec 31, 2002 at 11:35:12AM +0200, Jukka Marin wrote:

> You can only "hear" one direction of the phone conversion because the ISDN
> S bus has separate separate signals for transmit and receive data.

With two (passive) cards and a special cable you can do that.
See isdntrace(8) and look for -a.

I've used a laptop with two pcmcia cards to sniff my test machine 
talking to the exchange while debugging ISDN code a lot.

Martin