A few minutes after the concert began, a backstage assistant called in sick who was responsible for checking whether everything is going well according to the timetable.

called 是动词 call 的过去式，意思是"打电话"或"呼叫"。 in 在这里是副词，与 call 搭配使用，表示"通知某人到场"或"召唤"的意思。 sick 是形容词，意思是"生病的"或"不舒服的"。 called in sick 是一个固定短语，意思是"打电话请病假"，指员工因生病而通知雇主无法上班。