If things don't work well,  first start with examining the logfiles in  /var/log/.

If you see ISDN codes,  for example E0010,  in a logfile,  you can find their meaning with the command:

man isdn_cause

You will see the code E0010 frequently.  It only means that the link is disconnected.  If you can see some lines before remote hangup,  chances are that you have entered a wrong loginname-password combination or telephonenumber.

When using HiSax cards, you also find logmessages in the logfiles /var/log/isdn.log and /var/log/isdnctrl.

The contents of /var/log/isdnctrl is also displayed on tty11.  If you did not start X,  you switch to tty11 with Alt+F11.  In the other case you switch with Ctrl+Alt+F11.

Because the ISDN link is actually a networklink,  you can also get usefull information by using the common network-tools ifconfig, route and netstat.

The script isdn uses the programs isdnlog, isdnctrl and ipppd.

That's why when you are having problems you should also consult the manuals of those three programs to figure out what other possibilities there are.

You can also read the FAQ (Frequently Asked Questions) in /usr/doc/ and on the Internet.

The following URLs are a good starting point:

http://www.wurtel.demon.nl
http://www.isdn4linux.de