If someone is calling you : "Who is calling" like "Who's that?" If your friend is calling to someone and you stand near him you would ask him "Whom are you calling"
Let's say that you want to speak everyday American English, not write a formal paper. In that case you would not go far wrong to use "whom" immediately after a preposition, "who" otherwise.
Since the sentence initial position is never after a preposition (by definition of "initial position"), never start a sentence with "whom". In everyday conversation we even avoid the cases