call it or no call?

[EMM]

How about this one:

Player A shoots and Player B goes to block the shot. The whole gym hears a slap of the guys forearm, but the ref says he doesnt see it, so he doesnt call the foul. He says he needs to see it not hear it. The ball clearly changes direction and never reaches the backboard and goes out of bounds. Ref calls the ball out on Player A the shooter. I, not being a ref but being a coach, would think that it is rather obvious that the defensive player hit either the arm or the ball which should have resulted in a foul on Player B or it being out of bounds on Player B.

However, the response from the ref is that he didnt "see the contact" of either the ball or the arm.

What is the right call here?