[just another ref]

Around here, neither the horn nor the whistle is absolutely necessary. I think of the horn as a way to get the officials attention, and the whistle gets the attention of the waiting sub (come in) and the other officials. (don't put the ball in play yet) If I have the ball in my hand and I am close to the table, if I see the sub get up and come to the x, I may wave him/her into the game without a horn or a whistle. The biggest problems here are: 1. Coaches who think that they can yell "Sub!" and stop the proceedings anywhere short of a live ball, even if the player is not even off the bench yet. 2. Many players at all levels think the horn is their green light to enter the court.