I set out this offseason to master tube, trumpet, and scratch box. The trumpet has been the most difficult for me. My tips are:
-back pressure is required either by covering part of the bell or making a chamber with your hands with limited opening or a combination of the two
-the idea of kissing with your lips will get sound but not really what you are looking for, I started sounding much better when I relaxed my lips and drew in much less air
-when you learn a mouth call you should exaggerate the yelp starting with the highest note and slowly dropping your jaw to get the lower note on the longer reeds, similarly with a trumpet start out with a nice high clear note holding it as you drop your jaw it will break over to the lower note and this is enhanced with a sound chamber formed with your hands around the bell
-clucks and yelps are shallow draws of air into your mouth while the kee kee is drawn into your lungs
-I will let you know about purring and gobbling if I figure it out...lol
I have never hunted with trumpet but have a ton of confidence I will call up birds with them this Spring. They are ultra-realistic in the woods. Look at Trumpet Thumpers forum in the 12' Contest forum. There is a tutorial video from Savduck describing a silly sounding method but it works. Good luck.