At a gym there are two courts seperated by a retractable partition. When one scorer's box is unplugged a horn sounds on both courts. Team A is dribbling up the court when the horn sounds and the player stops and begins walking the ball to the ref as if he has made a call. The ref calls traveling on Team A and the ball goes to team B. Is this the correct call?

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That is why coaches tell players to play to the WHISTLE. Correct call, but certainly not a popular one I would imagine.

Better answer might be to give the ball back to A. But it depends upon how things were going. Violation was the rulebook/correct call.

As soon as I see the player pick the ball up with the horn, I'm blowing a whistle. Then it's "no violation- team A's ball" afte I check the horn out. If I can, I'm just not gonna wait to see the player commit a violation.

Even I don't blow my whistle fast enough to be right after the horn sounds, I am giving it to A because I'm sure I will learn from somebody what happened, and giving it to A is the right thing to do.