Two answers... If the bulls are still alone and haven't gathered cows yet, they are still in the strut and bugle stage. The pecking order hasn't been established and they will probably respond better to a challenge than they will to a proposition.
Lots of tree raking and bugling. If they scream at you, scream back. If what you hear back sounds more like a territorial bugle, then just try not to sound bigger than the bull you hear.
If the bulls have gathered cows, a cow call will garner responses from smaller satelite bulls and the occassional herd boss who thinks he needs more girl friends. If you challenge him, chances are about 50-50 he will take the cows he already has and sneek away.
Sometimes, if you can get close enough to a herd bull without getting spotted then bugle, you can make them think there is another bull already trying to run off with his cows.
So play it by ear and see what the situation is when you get there.
I just noticed you will be hunting in AZ. What ever you do, find water. As dry as it is right now, if you find water, you will find elk.