Here are links to a few great how to videos on using the mouth call. Scott Ellis is regarded as one of the best or possibly the THE best mouth caller in the nation.
Cluck and purr
http://youtu.be/uL-lfYCuzEQ
Yelp
http://youtu.be/LkKR3eYnBd4
cutting and excited yelping
http://youtu.be/ej3sAAzAdpw
Cluck and purr
http://youtu.be/uL-lfYCuzEQ
Yelp
http://youtu.be/LkKR3eYnBd4
cutting and excited yelping
http://youtu.be/ej3sAAzAdpw