Let's say I get a wrong number and I want to politely tell them that it is the wrong number. To do that, can I say/ask
What number are you trying?
Who do you want to speak to?
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Both the answers you suggest are polite, and don't divulge your identity in the process. I'd probably phrase the first as
What number are you trying to reach?
If the person on the other end doesn't realize at once that they've reached a wrong number, I'd follow their answer with
I believe you've reached a wrong number.
and that usually finishes it.