Uwe, Questions always welcome.
You must suck, not blow, to make the yelping sounds. It is a rhythmic sucking, like lighting a pipe, or kissing repeatedly. In the "How Tos and Build-a-longs" forum I just posted a compleat teutorial on building the calls, and near the begining, as I recall, is a more extensive description of the use or technique. I hope this helps.
One of these days I must learn how to post a video/audio clip and add it to my teutorials, but I just stay too busy (I think - LOL).