FF, entire volumes have been written on this; I would encourage you to seek out a few. As for me, my turkey expertise is sadly limited to "book smarts", despite having a go at it a couple of times.
The important thing is to positively identify the game is in your AO in the first place, something I haven't mastered yet.
As far as some modest specifics in calling, from what I've read, it largely depends on whether you have positively identified a roosting tree, and how far away you are in the AM, which is the best time to nail 'em.
If you have, starting out with a gobble gets the gobblers excited, being fearfull that somebody else is down there already getting his hens!
If not, starting with a few clucks and purrs to lure one in that hasn't found his yet is the flip-side of that advice.
But to borrow a tried and true sage saying from Fort Benning, "It depends upon the terrain and situation".
