I didn't realize they made such a thing. I learned how to call the cows from my dad. I don't really know if it makes much difference what kind of sound you make as long as the cows can associate it with food. Try hollering something out while you are putting hay or something out for them. Once they associate that sound with food, it won't matter if you are actually feeding or not, they will come. We still use this method when moving cattle between pastures. Drive to the top of the hill and yell "Soooook cows" and then take off towards the gate, they come running and somtimes pass you up once they figure out they are going to fresh pasture.