The trick is not to scare them off too soon. Scent would be number 1. Second would be having been hunted before, heard the call before, or seen ya. I honestly had no luck with e-callers, but you could start out with it then switch to a mouth call to get them closer.
But I must also admit that I never tried the high end e-callers
Navin R. Johnson: "He hates these cans!!!! Stay away from the cans!!!!"