Using a trumpet call is done the same way as a wing bone. I use alot of Fred Cox's calls and love them.
I requires alot of practice, but I like to refer to it as kissing the call. You will have to find the right place on your lips that gives you the best pressure, as well as the best angle in which to hold the call. The way in whick you cup you hands will greatly effect the pitch as well.
The above link is a good one, but you will really just need to practice practice practice. Everyone is different when it comes to these calls so you will just have to find your nitch.