'It is I' or 'It is Me'?
Pretend with us, if you will:
You're calling a queen. You two are chummy, and you have the number that goes directly to the telephone located on the table next to the armchair in which she is sitting and awaiting your call.
She answers and says, "Hello?"
You say: "May I speak to the Queen, please?"
She replies, "This is ___."
And there we will stop: what does she say?
She might choose to answer with her title: "This is the Queen." Or she could go the pronoun route.
Being queen and all, that might mean going full royal: "This is We." Or would it be "This is Us"? (We're assuming the caps, but that's really up to her.) But what if she prefers a plebeian (and lowercase) pronoun? Would her answer be "This is she"? Or "This is her"?
It all depends on how she regards that little verb is. Such a common verb, but even queens (English-speaking ones, anyway) have to use it. Its infinitive form is be, but it of course has other forms too: am, are, was, were, being, been. Be is the most common of the linking verbs (also called copulas or copulative verbs). A linking verb is a kind of verb that, instead of expressing some kind of action as verbs like "run" and "digress" do, connects a subject with an adjective (or adjective phrase) or noun (or noun phrase) that describes or identifies that subject. For example, in "The Queen is waiting for my call," the linking verb is connects the subject (the Queen) with a phrase that describes the subject in her anticipatory state.
For a long time, grammarian-types asserted that when you've got a subject that is followed by a linking verb, the thing that comes after the linking verb (the adjective or noun) should be in the nominative case—that is, in the form that is used in the subject position. We can think of the assertion like this: a linking verb is akin to an equals sign. Just as we would say "She is the Queen," we must also say "The Queen is she." There's a fancy grammatical term for this: predicate nominative. It refers specifically to the adjective (or adjective phrase) or noun (or noun phrase) that follows a linking verb to complete its meaning and is required to be in the nominative case.
Most of the time we don't have to think about whether what follows a linking verb is in the nominative case or the objective case (the form used in the object position). In "The Queen is very funny" and "The Queen is an excellent conversationalist" the adjective and noun phrases following the linking verb have the same form whether they're in the nominative or the objective. But when we want to use a pronoun after the linking verb, we must make a choice.
If the predicate nominative holds, the Queen will say "This is she" (or "This is We," if she's going with the vaunted pronoun that sovereigns sometimes employ). This is connected via the linking verb is to the pronoun that identifies the speaker in the nominative case. If the Queen answers instead "This is her," she is denying the predicate nominative and treating the pronoun that is connected via the linking verb is as though it were coming after a regular old verb such as like, as in "I like her."
And what about the rest of us? Should we deny the predicate nominative, or embrace it?
The answer is, we assure you, purely a matter of style.
While there was some heated debate about the matter in the 18th century—mostly a single it is me defender was quickly outnumbered by some influential it is I people—by the early part of the 20th century the majority of those who make recommendations about such things were acknowledging that it is me is perfectly fine, especially in informal use. Both forms have existed for centuries, with it is me tending to appear in more relaxed contexts even long ago. Which means you—and the Queen—can choose whichever you prefer whenever you like.
The predicate nominative of course comes into play with other pronouns as well, and when it does it often sounds particularly well-suited for the regal among us: "If I were he …"; "I heard a knock—it might be they …"; "hoping it was she …" Without the predicate nominative we have "If I were him …"; "it might be them …"; "hoping it was her …" Again, the choice is up to you. As for us, we reserve the right to save the former exclusively for our confabs with the Queen.