I've called many coyotes in with nothing but repeated howling, never ever been barked at. Much like a continuous squealing rabbit, which doesn't really make since in my mind, repeated howling also doesn't make since but it works.
Another thing that in my mind shouldn't work, is having single coyotes come to a serenade, but I've had that happen several times too.
Been many MANY years ago that I witnessed a single coyote howl over and over and over again only to eventually have another coyote come to it. From that day forward I knew that howling can and will call coyotes.
And I'm not saying I turn on my favorite howl and let it run until they show, but I will howl several times and then spread it out and not get into a pattern, but howl repeatedly. Once I see one coming I then only howl if it checks up, and usually a howl or two will get it started coming again. Done it many times.