Well-made scratch box call
A good box call
A mouth call that works well for you.
Those are my three main calls I carry, and can do soft yelps on all of them...
Heck, If I think I can do it without scaring them, I hold a box call over my head standing up, and do a soft tree call... If I use a wing for a fly-down, I try to bring it from a high spot to a low spot.
Birds sound different on the ground to me than in a tree, and I have to think that it does to turkeys as well. And, I have fallen asleep behind a log or cover, only to be dramatically awaken by a gobbling bird right beside me (that I called to some time ago), so I know they can pinpoint sound and location quite well.
More and more, I call less and less at birds on the roost though...