In the Season 1 episode, Health Care, Jim locks Dwight in the conference room which Dwight has claimed as his workspace. Dwight yells, however, no one helps him. Thinking his Assistant to the Regional Manager position has much power, he calls Jan Levinson to tell her that he is firing Jim.
Jan tells him not to fire Jim because he has no power when Dwight spills that Michael gave him the power to change the health care plan. Jan, angered, hangs up and later calls Michael. Jim doesn't let him out.