isdnconfig can't hangup if it didn't dial.
If isdnconfig is started when an isdn connection already
exists it shows the current state as connected and has
the Hangup button enabled. However clicking Hangup
has no effect. No error dialog is produced however if
isdnconfig is started from a terminal then isdnctrl produces
its "usage message" to stderr.
*** This bug has been marked as a duplicate of 17973 ***