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 ***