I have a black Primos crow call that pulls gobbles pretty well, though I don't use it to imitate a crow. I hit it hard with two short, loud blasts. Turkeys hear crows all the time, but they don't gobble at all of 'em. I use it more scouting or at dusk to pull a shock gobble and see what's where. It works like it means it. (kudos to Gobble Nut for the technique
).
Jim