I use bugles to locate bulls. When I locate one, i typically start out with a few cow calls when i move in very close to where I think the bull is. Like many said above each situation can be different. Some bulls roar like a t rex to the first couple cow calls, and come charging in. Some scream their head off wanting you(the cows) to come to them. Some could care less about the cow calls, and only respond to the bugle. Its easier to add other stuff to the cow calls and add some bugles in if the cow calls didn't work after you try just a few quet cow calls first.
I had one bull that would only answer a series of cow calls, then a bugle followed by some light chuckles. You had to do that series of calls 3 or 4 times then he would chuckle and bugle very quietly. I tried all kinds of other stuff to get him more aggressive and nothing worked at all. I just went back to what worked, and kept on him.He was moving so it made it hard to get in really close. After about an hour of following him across a basin we finally got what sounded like 80 yards away from him in the in the timber. It took another 10 minutes doing the sequence of calls and my buddy shot him at 35 yards.
I have called in bulls with just a few cow calls, bugling as loud and aggressive as possible, just raking a tree with no calls at all, cow calling kicking rocks raking a tree and an occasional quiet bugle, making cow and calf sounds with a loud aggressive bugle, cow calling kicking rocks raking a tree and bugling as loud as i can cutting the bull off when he bugles, and probably just about every other combination of sounds possible.
It does seem to really piss off a fair number of bulls if you copy what they are doing, but cut them off when they call. For example if your calling and the bull seems to only chuckle, switch to just chuckling. When he chuckles chuckle over the top of him loud. While you are doing your calling you should be able to hear if the bull is getting more wound up.
It really helps to hunt areas where you get more encounters if your new to calling, so you can learn to read their emotions, and figure what will make them come in.