For 70 acres, I would buy a whistle.
My horses share a smaller paddock, the entire thing is visible from the gate, but our small herd of cattle run about 30 acres of hillside, gully, and woods. It's amazing how 20 1500 lb. Cows can vanish in dense forest.
Our auditory call of choice for them is either a loud "HEH! HEH!" (think a contracted form of "HERE!") through cupped hands, or to just fire up the 4-wheeler that their grain "treats" and mineral blocks roll in on.