I have yet to master the mouth calls. I keep trying though but usually it's spit-tooey and it flys out of my mouth or I have too much spray.
My favorite is the box calls followed by slate. The box calls give a richness to the tone of the calls that can't be duplicated with a mouth call. With my box calls I do not use chalk. Instead I use rosin to prep the call. That alone gives a richer tone than chalk.
On my slate/glass calls I rough them up good with sandpaper then apply some rosin to those calls too. It waterproofs box and slate calls so that they work when wet. Chalk won't do that.
Now if I could just glue that mouth call in so that it will stay in place. I also need a splatter guard too
If you really want to learn go on a guided hunt and sit back, listen and learn when the guide does the calling. Nothing better than an outdoors classroom.