I was able to call in and arrow a bull, with a spike squeal, using the mouthpiece to my Power Bugle once. It probably wasn't the most convincing tone in the world, but the timing was right, this particular bull had a single cow, and he didn't want any other bulls near her. He came on the run from 150 yards away, and stopped in front of me at 20 yards, stomped his foot, and let out a little huffy squeal! Turned broadside, and he was all done.
I'm pretty convinced that a full blown challenge bugle might have run him off, he was not a big bull. I just happened to make the right sound at the right time.
It's hard to generalize when it comes to calling elk. Adapting to the individual situation is key. Most of the time, it seems to be more about the right timing, than making the perfect call.
Best of Luck,
Jeff