It largely depends on the country you are in and what kind of animals you might be calling. Calling is fun, but I don't really like doing it unless someone is with me. In the areas I like to hunt, the following is a list of the animals that might be interested in a distress call; fox, coyote, black bear(s), mountain lion(s), wolf(s), and grizzly(s). That is too much fun for one guy. Until you see a bear come in to a call, foaming at the mouth, you don't know what you're made of. You had better be wearing some sort of hearing amplifier and have a machine gun.
Hunter's Specialties produces a video that is pretty good. It is called, Call'n Bears, They Com To Eat!