Well AS, don't expect to call one in every time! But when it happens it is exciting!
With that howler being an open reed call it does take a bit to learn to play it!
The 1st thing I did wrong was try to blow to hard. Blowing harder changes the pitch. Grip the call about mid way down the reed with your lip on the bottom and your teeth on the top. Hold the call up at about a 45 degree angle.(changes the pitch and holds it tight to your teeth) Slowly start to blow on the call and slowly slide out to the tip while keeping pressure on the reed.
Barks can be done by quick breaths on the call. Sort of making like a "PA" sound. You have to tweek it a bit, its not really the same motion to make a "PA" sound but that will get you started.
Now PRACTICE, PRACTICE & then PRACTICE some more!
A nice closed reed call will make a nice distress sound with a lot less learning curve. But mastering an open reed will allow you to make almost every sound!
Good luck!