I place my calls in the roof of my mouth (obviously) but I try not to manipulate my tongue very much. Meaning, I let my tongue lay where it would ordinarily. Obtaining the rasp is attained by running the call while ever-so-slightly "squeezing" the call up against the roof of my mouth with the back part of the tongue. This works for me but may not for you, but keep practicing and you'll have an "Ahaa!" moment.