I've had my best luck with just chirping or a locating call on a reed, moose grunts work well ,too. I don't chirp a lot, couple or three times in 1/2 hr. Locating call can bring a bull from quite a distance, if u get an answer, try a loud chirp back, add another short locating call. Then I stay put and shutup until I can hear or see the bull, I will chirp again in about 1/2hr if I haven't heard anything else. Very often they will come in dead quiet, or with real quiet chirps/grunts. Sometimes a screech on a bugle will get him PO'd, a bigger bull will usually answer a chirp and a short screech, but, that takes two people about 30-40 yds apart to work it. Screeches will usually spook a smaller bull, cuz they don't want to get wailed on. Bugles seem to make a herd bull gather up the girls and exit,unless you are in the cows,in which a screech will usually get him really torqued. It's tough to get there though, cows have radar, sonar, spy satellites, infra red,fleer and the wind will always change directions at the wrong time.
Had a buddy get sprayed on by a 5pt with the chirp & screech routine, he still stinks.