1. A work shift.
2. A time to report.
3. To direct others.
4. A decision.
Most often used in conjunction with a modifier: half-hour call, curtain call, casting call, focus call, set call, show call, rehearsal call, photo call, call time, call cues, call back.
See also the thread: http://www.controlbooth.com/forums/question-day/14038-call.html.