list trace station <extension>
This will show you all of the activity for the station. You can have the user place a call, and see which line the call used to go outside.
The report is pretty detailed, and is likely to scroll right off the screen. Let it run, and let it keep going page after page. When you're done, enter the command:
list trace previous
This will re-play the last "list trace" command from the beginning so you can follow it more slowly.
Carpe dialem! (Seize the line!)