在不清楚对方是谁的情况下用it 还是who

用IT
who is it that knocks the door?
I don't know who it is.
maybe you should go and check it.

另外，电话的另一边，婴儿都用IT
希望对你有帮助

这种情况下it做的是形式主语，而who是从句引导词。

其实你这句话用who就行：I don't know who is at the door.
如果说没有at the door，那也有who和it都用的情况：I don't know who it is.

给你给最简单最容易理解的EXAMPLE：
电话响时：
A：Who’s that ？
B：This is B speaking.
当你不知道对方是谁的时候，要用It、that、this 来代替.

用IT
当别人不知道你是谁会问 who is it?

这时你应说 it is XX

一般不知道是男是女的情况下用it，I don't know who is it at the door.

肯定是it