Good advice from Rory.
A locator call is exactly what its name implies... something just to make a bird gobble and give away his position. Lots of useful locator calls acctually come from nature and your natural surroundings. For example, there was a place I hunted when I lived in SC that was in relative location to a fire house. Every morning, at 730 sharp, they would roll the engine and the ambulance out and do a quick "siren check". No finer locator call in the world... every morning you'd hear five or six in any direction sound off... any day of the year... no matter the weather.
I've had them gobble at closing car doors, flocks of geese, car horns, freight trains, and they really go off at thunder. Crows, owls, coyotes, hawks... sometimes with young birds that have apparently just learned how to gobble, you can acctually yell obscenties at them and have them gobble back.... not exactly productive for hunting... but extremely entertaining
What call you use really depends on where you are hunting (geographically). Most of my hunting is done in my native VA, so coyote calls (though there are yotes around here) isn't really productive. If they are in the mood, they will gobble at it though.. just like anything else. What really works in my part of the world is the owl call. Usually when you start up with it, you'll get a real owl to respond, then another owl to call back to it, and pretty quick you have a whole parliament of owls going off. A turkey can only take so much, and if it doesn't work the first time, he will eventually gobble, perhaps to just say... hey keep it down I'm still trying to sleep. As more owls call in different places, eventually one will be close enough to a turkey to make him respond. Owls and turkeys are kind of like Red Sox and Yankees or cowboys and indians... not the best of friends. Sometimes a turkey will gobble from several hundred yards away, but typically you need to be inside of 250 yards or so. Depends on the conditions, and the time of year. As spring rolls into summer and vegetation immerges, your calls will be muffled and you'll need to be closer.
Owls of course, work at last light and first light.. so blowing the owl call in the middle of the day is not advisable. Once I hear the first real crow of the day caw.. then I know its time to switch from the owl to the crow call. Makes sense to me anyway. If you hunt near a river, and have geese around, then bring the goose call... this has been a real ace in the hole for me over the years.