To call to a person a long way off, you project your voice with the intent to communicate clearly which gives it ample resonance. Yelling, however, is stuck in the throat because the emotion that produces it is out of control. Calling - on the other hand - is vibrant and cool under the collar. You want to connect with someone, not bludgeon them with sound!
Why is calling the subject of this post?
Calling was used by the Old School to open the throat of the student. It is an auditory phenomena rather than a mechanically imposed action. You don't open your throat to call. Rather, it opens when you call- a very different thing.
Calling joins three aspects of historically oriented vocal function together in one seamless action.
2) open throat
Try calling on [i] or m[i] and you'll get an idea of what resonance/projection is. If you hear it buzzing in the center of your head, clearly in the front of your mouth and in the 'bubble of sound' around you, you're on the right track. If this is stable and successful, practice calling on every pitch and dynamic level, sliding up and down without loss of clarity or buzz.