On the telephone, the person answering it may say "Who is calling?" or "Who is this?" Why do you say "this" to the caller on the other end of the phone? Is is wrong to say "Who is that?"
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You would say "Who is this" because this is a Predicate Nominative and shares the same context as the subject. For example, both "Who is this?" (Subjective) and "Whom is that?" (Objective) are correct. Also, "Whose is this?" is also correct (Possessive).