Wiktionary
vb. (context transitive English) To gather (especially people or dogs) together, calling them by whistle.
Usage examples of "whistle up".
She picked the whistle up cleverly in her two front paws, juggled with it a little, then let it drop.
The tide, he had observed, was out and it seemed highly unlikely that the authorities could whistle up a battleship at such short notice -less than two hours.
And then, she thought, the customs agent standing by in the jetway would whistle up the men in the white coats.
Sanborn was almost dozing off when he heard the sound of a train whistle up the line toward Plymouth.
Would I have ridden into these hills in the Big Dry, I who cannot whistle up the water, were it not a matter of medicine?
She could bring the most powerful creatures to her side as easily as a shepherd might whistle up his dog.
They huddled there helplessly under the sidewalk canopy along with everyone else, unable to move, while the theater doorman blew his whistle up and down the street--that querulous sound just then, chiming in upon her thoughts from somewhere in the distance, reminded her of that day, brought it before her more vividly than ever--calling up carriages and cabs and anything on wheels to the rescue.
You could hear a straggler trying to whistle up somebody all the way down at the next corner.