Early in the morning he can hear you almost as far as you can hear him.If you are 400yds or less I gaurantee you he hears you.Just because he doesn't answer doesn't mean he doesn't hear you either.Later on when the wind,song birds..etc get going it will cut the distance down.Also depends on the foliage.Most likely if he gobbles when you call he is hearing you.Don't call to him every time he gobbles.If he responds wait 10 minutes or so,then call again.If you call too much he will just sit right there gobbling until he finally loses interest,and walks away,predator spooks him,or a hen steals him.