Most calls gets their attention and has them looking. That includes the two quarters. However, they seem to lose interest very quickly and go on about their work. I have used the whistles, quarters, Barkers and a few homemade gadgets. What you really want, is something that really ticks them off and gets them to start barking at you. This happened one day while crow calling and every time I blew it, they started barking. I had crows coming in and squirrels barking til I shot the first crow. Another time I had a call that sounded like a cat meowing and that got them pi**ed off as well. You can also rub two walnut hull together and they will look. Practice in your back yard and like I said, they lose interest very quickly. i also get their attention by waving an old tail or rag like a tail on an excited squirrel and again, they will come and look. Sure hope none of you fellers hunt in my woods cause you are likely to call the boys in white suits.
Be Safe !!!