If you can gargle you can purr with a diaphragm. Practice without a call to begin with. You will have to learn how to close or tighten your throat and make a high pitched growl. This is how natural voice callers purr. When you can do this place your call in and apply pressure with your tongue until you get a high pitched purr out. The other method is to purse your lips as in whistling and let you lips flutter as the air is exhaled. Hope this helps.