To be is a verb in a troublesome spot.
It wants subjects for objects. That’s weird, is it not?
When subject and object are joined by to be,
The object’s a subject: I am I; he is he.
The subject and object are reversible. Oh, dear!
I am sure. Sure am I. Here is Tom. Tom is here.
So, to answer the question, “Who’s there?” you reply,
“It is me,” very meekly or, with guts, “It is I.”
Or, when the voice on the phone asks for Millie McGee,
If that’s who you are, you reply, “This is she.”