I have called in several bears in Washington State.  16 so far.  I like to use a loud raspy mouth call.  I start off quietly and increase the volume after a few minutes.  I call in a bear about every 20ish stands.  I call non-stop for at least 45 minutes and try to make it 60 minutes.  I have the best luck, obviously, when I have a lot of fresh sign, but do a lot of cold calling.  I believe if you hear the bear coming in, most often the bear is a larger bear.  I think they are trying to intimidate whatever is eating whatever is screaming.  Most mid-sized and smaller bears sneak in because they don't want to rush in and get their butts kicked.  I had to pepper spray a small sow several years ago. I was calling and heard some pine needles crunching behind me and slowly turned and about 15 yards behind me was this small sow.  I did not want to shoot such a small bears so I softly spoke to her and she stepped forward.  I stood up and said hey bear.  She pinned her ears back and started towards me so I sprayed her in the face and she left.  Another time I was calling with a buddy and his young son.  I was calling, slowly turning my head looking and there about 10 yards in from of me was a pair of beady eyes and a pair of ear looking in my direction from about 15 feet.  All I could see was the top of his head over the ferns.  I kept calling and raised my gun and shot him between the eyes.  Scared the crap out of the guy I was hunting with.  A few times calling I never heard the bear come in, but would catch movement and watch them turn and walk away.  It is very effective, but you have to stick with it.  I think most guys are not successful because they give up too soon and don't try it enough times.  I like to put my watch out in front of me, so I can keep track of the time, because calling for that long, feels much longer.