'Whom' vs. 'Who' at the Beginning of a Sentence
"Whom was called into the office?"
Technically, that "whom" is correct because it's the object of the verb "called." Yet almost no one would say it that way. Does that mean everyone's wrong? No. It means that, when the pronoun's at the beginning of a sentence, even the most formal writing can use "who" as an object. Here's the full explanation.