If they can see me, I use a high - low whistle. If they are too far, I use the horn of a car or truck. Had a pony mare that liked to drink whatever we were drinking. If they were close enough, I'd just hold up my go cup (usually iced tea) and she'd come running with the others following.
I worked on a ranch that had broken horns in every truck. Every one. The owner was in Denver and hit a few pawn shops buying up small brass instruments. Coronets, trumpets, bugles. Every truck got one, and some of us even learned to blow a tune or two on them to call in cattle and horses.