Roll your tongue without the call in your mouth. Roll it loud and long until you can change the pitch of the sound that comes out by tightening your lips. Keep doing that, really loud. Now put the call in and do the same thing, really loud and long rolls with the call in. If you can't do it, take the call out and do it some more, then put the call back in. Once you finally get the loud purr sound with the call in your mouth its time to make it sound like a turkey, so practice on softer, shorter, and more controlled purrs by tightening your lips and adding or removing pressure with your tongue. Just listen to clips of real turkey purrs and practice, but of all the techniques to purr on a mouth call, I think this is the easiest way to get a realistic purr.