You could try to isolate the stations who typically make these long distance calls by assigning them different CORs.
Then on the "change cor x" screen set the "Outgoing Trunk Disconnect Timer (minutes):" field to "15". Of course, this would mean that any calls they make while they have that COR will automatically be terminated at 15 minutes, whether they are long distance or not. This may or may not be a trade off you are prepared to make.
Hope this helps
Nick