Q. I answer our company’s main phone line, and frequently get calls for myself. Today when someone asked for me saying, “Is Charlotte available?” I responded, “This is she.” The caller promptly corrected me, informing me that I should have said, “This is her.” Which is correct?
A. Your response was the correct one. “This is she” is grammatically correct. The verb “to be” acts as a linking verb, equating subject and object. So this is she and she is this; “she” and “this” are one and the same, interchangeable, and to be truly interchangeable they must both play the same grammatical role—that of the subject.
However, this rule gets broken all the time. I suspect that people expect an object (as is correct for constructions such as “you slay me” or “what’s wrong with me?” or “go talk to her”) so they choose an object, unaware of the nature of a linking verb. Now both forms have come to be accepted if not acceptable; it’s a matter of how formal you want to be. If you’re a 1950s-style Hollywood garage mechanic who grudgingly picks up the phone, with greasy hands, when nobody’s “manning” it, the conversation might go like this:
That’s me, Mac. Whaddya want?
You can try to avoid the issue by using your own name, rather than a pronoun: “this is Charlotte” is never wrong.
The who/whom question is similar. Though “whom” is correct when the objective form is called for, it can sound put on; it seems to draw attention to its own correctness. In any case, if we were all as proper as you are, proper grammar wouldn’t sound wrong to anyone.