According to grammar rules, it should be this is she, because is is a linking verb (a verb that connects the subject to more information about the subject), so it can't have an object (her), but it's. Upon answering the telephone, the person calling asks if joan is available. If joan is the person who answered the phone, should she say this is her or this is she?

If you are an actor in something, it's in: She was in cat on a hot tin roof. She was in the movie cat on a hot tin roof.