You can google Mallard duck calls and you will have more choices than you can imagine. I doubt there is a call that will make the birds come out of a game reserve or refuge . If they have food and shelter there is no reason for them to eave where they are. There are no magic calls that will make a hunter a pied piper. Ducks respond to a call when they are looking for company, I suspect they have all the company and everything else they need inside the refuge. Look at it this way, if you were inside a safe comfortable home, you have food, drink and lots of companionship of both women and men, would you respond to an unknown person outside that you cannot see or know shouting , trying to get you to come outside?