I'll start by saying that some of the guys on here kill more in a weekend or good contest than I usually kill in a year. So keep that in mind. I started 7 or 8 years ago and the learning curve was tough for a while but I'm gaining.
But, the past year and especially so far this winter I've had a lot of vocal responses to introductory howls. And a lot of those have come in. I was always a bit leery of starting off with a howl, until this fall I watched a totally disinterested coyote at 600 yards yawn and nap while I was calling. Finally I just wanted it to stand up so I threw out a couple lone female howls, it not only got up but made a bee-line on in.
So nearly every stand this year has started with a howl. Here's the thing, if something responded it has depended on how close they are if they come in or not. If they are mile and a half away, no probably not coming. But use that info to set up the next stand. If you can tell they are within half mile, get ready IME. If I get a close, immediate response, I have gone straight to pup distress and it seems to draw them in. The other day it was really close, like 1/4 mile and I went to lip squeaks and had 3 in my lap in a couple minutes.
I've had 3 or 4 that responded from 1/2 or 3/4 mile with challenges. Figuring those were bigger males they wouldn't be afraid of a fight so went to an aggressive fight sound. In each case that shut them up and I figured they were headed in, and ended up killing them. If they had kept challenging my plan was to challenge back.
So any more I like to hear a response, if nothing else it helps me plan the next stands. But don't let a lack of response discourage you either, Monday I got 2, and didn't hear a single coyote all day.