Practice makeing the tone and cadence.  I force air up from my diaphragm versus just "exhaling" I also, try to get the echo sound by letting my cheeks expand.
As far as voice yelping, I may not be the best, but I have called in and killed turkeys by doing it.  I tighten my throat to get the higher pitch and somehow I get a little rasp, too.  Maybe I should record my voice calling so you get the jist.