"Who is it by?"
Being new to Italian, it has been interesting to me to see many questions using the same word order as a statement, but with a question mark at the end to indicate that it is interrogative. Why is it not acceptable in this case to ask, 'E di chi?' ? (Sorry, my English keyboard doesn't let me put the accent on 'E'.)
Don't worry, even Italian keyboards don't have the capital "È", and it's become accepted to use E' in its place. When asking this kind of questions, who/what/why/when/where/how, the adverb is at the beginning of the sentence, preceded by the preposition if needed as in this example. For instance "Dove sei?" (where are you?) instead of "sei dove?" (you're where?); the latter gives a sense of urgency, just like in English.