In English of course answers is both the plural of the noun and the third person singular form of the verb answer (which is what we have here). But in Swedish, the noun is ett svar, svaret; svar, svaren, and the verb is svarar.
No, firstly you can't say "vem det finns" and secondly the use of finns sort of implies a question of actual existance. You could say just vem där? for asking "who's there?". Or if you're on the phone and want to ask a little more nicely, you could say "vem pratar jag med?".