We say “Who is it?” any time to any visitor including salespeople. 「dochira sama desu ka?」「dochira sama?」「donata sama desu ka?」「donata sama?」「donata desu ka?」「donata?」 The tone maybe changed to unwelcom people. (Or only 「hai?」to respond to the bell and wait for the visitor to speak who it is) Kids and teenager say it casually rather “Who are you?” like 「dare?」「dare desu ka？」 Only some kind of rough people say “Who are you? 「dare da?」 So generally there is no chance to say “Who are you?” in daily life. But you can find it in something such as movie, drama, anime, game, etc.

When someone rings your door bell and you don't want to be bothered by salespeople, the both "Who are you?" and "Who is it?" are OK, but, how you say it is more important. If you say "Dare desu ka?", it's very rude. If you say it to the people you met first time, it's really offensive. "Donata desu ka?" or "Dochira sama desu ka?" is normally used in Japan. "Dare" and "Donata" and "Dochira sama" are same meaning "Who". "Donata" and "Dochira sama" are more polite than "Dare".

IN Japan you don't say "Who are you?" to somebody. It's kinda rude. You find out by introducing yourself first. The other guy will respond automatically. Who is it? = Dare da?

If your friend told you she was dating someone, you wouldnt say, who are you? you know would she is, haha. you would say, who is it? as in who is it that your dating? but who is it is much more casual. so when someone is at the door ..... : knock! (you)who is it? *saying who are you doesnt make sense* (some guy)Mr. wierdo (you)what do you want? (mr. weirdo)I'm here to creep you out. (you)Okay come in! Part 2~ (walking on the sidewalk.. weird guy comes up to you) (weird guy)Hiiii!! (you)who are you? (weird guy)Jake from school! (you)Oh hey! whats up?