You could modify this in the Gameplay Constants. I'll find the exact function for you.
EDIT:
Here we go,
Advanced -> Gameplay Constants, scroll down, "Combat - Call For Help Range"
Also, in the object editor, set the "Combat - Acquisition Range" to something low like 100.
Beware: setting the "Acquisition Range" below the actual range of the unit will reduce its attack range.
Like, if you set Archers' acquisition range to 200,
they will shoot arrows up to a 200 distance despite their real range being 500.