The use of Singular they is very common I guess.
Then, Why is used "it" in "Who is it"?
”Who is it” is commonly used on the phone. I think i should use "Who are they".
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In this construction, "it" does not refer to a person: it doesn't refer at all.
It is a purely grammatical place-holder, like the "presentative" 'it' in phrases like "It seems that... " or "It can't be ... ", or "It must have been years ago", or "It doesn't make any difference", or thousands of other similar expressions.
There's no answer to "what does 'it' refer to?" in these, because it doesn't refer to anything.
You're not going to find many "rules" in English without exceptions.
This is one of them. This is probably the only time you can get away with using it to refer to a person.
Who are they
While third person plural pronouns are often used as a "gender neutral" singular pronoun, it doesn't work here. This will always sound like you are trying to talk to multiple people. Stick with who is it?.
Whenever it's possible it's better to avoid calling a person 'it'.
Look at this example:
- John, there's somebody at the door!
- Who is it?
- I don't know.
- Tell them to wait, I'm coming.
When somebody knocks at our door we often say, "Who is it?" or, "Who's there?".
Both aren't considered rude and are practically used everywhere.
If someone calls you on the phone and you ask "Who is it?", "Who's this?" it will be considered quite rude.
"Who is this?" is considered to be an idiomatic way to ask, "Who is on the other end of this phone conversation?" (or, much more stiltedly, "With whom am I speaking?") Yet, I personally consider it to be rude too.
It is better to say:
Or if in a more formal way then:
But it wouldn't be considered rude if somebody afterwards asks you "Who was it?" or asks you while you are speaking "Who is it?"
In most circumstances, you could say that 'this' is used as a way of 'pointing', either with your finger or just verbally. 'It' is used when there is no need to point because it has already been established what thing or person is being talked about.