I have a HiSax-based PCI ISDN card, loaded successfully onto the RH9's kernel (here's a part of the /var/log/messages).

This board is connected to an Intracom (intracom.gr) NT ISDN terminal;

Now, after defining an ISDN0 connection with this board, I cannot connect to any network, and I cannot even see any activity on the ISDN terminal.
The failure message in the log is:
I've tried to use KISDNdock, it **sees** (it's green) the board, but when trying to connect, says '""isdn0: resource temporary unavailable""

This ISDN connection is intended to make a permanent network connection (net extension) between two RH9 linux boxes, on a leased line.

My question is: how can I make all the preparations and tests, step by step,in order to find the real cause and the solution to this issue?

I had the same problem. Check this site:
http://home.t-online.de/home/Bernhard.Hailer/leafsite/

i've done something when i was trying this solution and this problem just disappeared

there's also an English version available

i'm not sure if it all works (i didn't manage to get my isdn card work) but imho it's worth seeing

thanks a LOT.
In the meantime, I've managed to convince the client to move to a permanent internet connection in all three locations (radio/optic fiber) and the problem dissapeared.

Anyway, I will check the site for future reference.