Source : WordNet®

wake-up call
     n 1: a warning to take action concerning something that was
          overlooked or neglected; "the bombing was a wake-up call
          to strengthene domestic security"
     2: a telephone call that you request be made a specific time in
        order to wake you up at that time (especially in hotels);
        "she left a wake-up call for 7 a.m."