"Busy" or "Just a moment" are probably better than "Can't Hear" if the goal is people waiting until you can focus on them and not them speaking louder/more clearly or trying to move closer so you can hear them better. Just from my personal experience of similar interactions.

But it's probably good to make people aware that 'can't hear' usually actually means 'give me a few moments to get to a stopping spot', if you try to remember to say something that indicates 'wait' instead of 'I need that repeated more clearly' and they try to remember 'sometimes that means wait', then hopefully it will work out more often than not.