Callers are used to attract their targeted species when they're not in stressed state. Some animals, may respond to your call by making a vocal response, e.g. an Elk may communicate by making a sound called a bugle. Each caller has a different attraction strength and range.
All callers can be purchased for 6000 except the Roe Deer Caller and the Deer Bleat Caller, which are available for free from the beginning of the game.