The only thing I have learned is that it has to fit your pallet right. I had to try about half a dozen different ones before I found one that fit right. Unfortunately that can get expensive. There are tons of different sizes & reed configurations
If it is too big, they can be trimmed. Start on the sides & trim tiny amounts until it fits.
Some people use different calls for different sounds, some can do them all w 1 call.
I feel it is like most stuff, you have to try a bunch until you find what works for you.
You can also find CD's or DVD's that can explain diaphram calling. I have a primos one, & I know there are others.
It is better to keep your mouth closed & be thought a fool, than to open it & remove all doubt!!