The umpire's responsibility goes from attack line to attack line. You are responsible for net violations, center line violations, substitutions, time-outs, alignment of the receiving team at the moment of serve, back-row attack, back-row block, antenna violations on the table side, supervising the score table (personnel as well as doing their jobs properly) and supervising bench behavior.
As Tim indicated, the umpire usually signals discreetly to the referee on a back-row attack or block. However, if, as Umpire, you are absolutely sure that a back-row attack or block has occurred, you can whistle and signal it if your partner doesn't.