You see this all the time in club matches but the RULE states:
USTA Comment 11.2: Must an out call on a player’s shot to the opponent’s court be made before the opponent’s return has either gone out of play or been hit by the first player? Yes. So, MT is right (finally)

Actually you can take a swing at it but if it is out, you better call it out before it goes out of play or your opponent whacks it.