Your phone is ringing and your daughter picked up the phone but she hasn't answered the call. Then do you ask her:
Who is it?
or
Who are they?
And when she tells you, would she say:
I don't know who it is"
or
I don't know who they are?
If none of the above isn't natural, then what is it?
The question (at least here in the U.S.) is:
Who is it?
It may sound impersonal, but it's idiomatic.
As for how one might answer that question, that could be done in just three words:
I don't know.
I'm not sure.
They didn't say.
(Or, your daughter could string all three of those together for a nine-word reply.)
As to how to phrase it in a longer form, I think you nailed it in your question:
I don't know who it is.
That's exactly how I would say it:
I don't know who it is – probably some telemarketer.
In modern English, the plural "they" is used as a pronoun in situation when we are not sure whether some previously discussed person is male or female. For instance "the average person thinks that { they are | he or she is } more intelligent than most others".
Like any pronoun, "they" needs an antecedent.
We cannot just us "they" out of the blue; we need to speak about some person first. For instance:
A: Someone is at the door!
B: What do they want? ["they" refers to "someone".]
Here B could also ask, "What does he or she want?".
B cannot say these things if A simply went to answer the door without saying a word.
Compare:
A: Hello? B, it's for you!
B: Who {* are they | is it | is calling}?
Versus:
A: Hey B, the caller is asking for you!
B: Who {is it | ?are they | ?is he or she}?
In the second example, B can use "they" because the pronoun has an antecedent in the previous discussion: the noun "caller".
"Who is it?" is most natural; "Who are they?" less so; and "Who is he or she?" is quite unnatural.
The reason is probably that "they" and "he or she" are restricted from use in WH- clauses which inquire about the identity of the subject. Consider:
A: Someone is at the door.
B: Who is it, and what do they want?
Here B speaks a sentence made out of two WH- clauses. The first clause asks about the identity of "someone" (who is that someone), and so the pronoun "it" is used to refer to that someone, rather than "he or she" or "they".
The second WH- clause asks about something else: what that "someone" wants. In this case, "someone" can be referred to as "they" or "he or she".
If the sex of the person is known, then this is not an issue. The correct gender of pronoun is consistently used:
A: Some man is at the door.
B: Who is he and what does he want?
B: *Who is it and what do they want?
Important note: A WH-clause asking a "who" question can be rhetorical. Contrast these:
A: Someone just called, asking questions like what is our household income.
B: Who {are they | *is it} to be asking that? It's none of anyone's goddamned business!
"Who are they to ask that?" is not a genuine identity inquiry but actually a remark that the person has no authority to ask such a question, regardless of who he or she is.
"They" is acceptable here, though 50 years ago, formal style manuals would have forbidden it.
Theoretically, one should use "He" or "He or she" when referring to a single person whose gender is unknown, but using "He" as a catch-all for women constantly leads to a subtle language bias, making it seem as if women are simply absent from everything, and using "He or she" all the time is tiresome. So "they" has come to be used as a singular pronoun when the gender of the person is unknown, even in cases where one is quite sure that only a single person is being referred to. "Their" can be used similarly. So:
Each student picked up his or her class schedule.
and
Each student picked up their class schedule.
Are now both acceptable, though in the past, the latter was frowned on.
In UK English (according to the Oxford Dictionary) it is acceptable to use the plural pronoun "they" in place of "he or she" when the subject's gender is not known. This was a 16th century practice that died out (somewhat), but it is now common and considered correct again.
In US English (according to Merriam Webster's Collegiate Dictionary) it is also correct to use "they" as a singular pronoun when gender is not known.
So, in UK and US English, all four sentences are equally correct.
(My experience living in both countries is that I almost always hear constructions 2 and 4 in the UK. In the US I almost always hear constructions 1 and 3.)
