I have been trying to get Pichu to call for help so I can get a Pikachu early.
If you lower the Pichu’s health into the red, it is five times more likely to call for help. Since Pichu has the highest of call rate numbers (15) it is very likely for it to call for help, especially if its health is in the red. However, in UsUm, a wild Pokémon will only call 1 ally without using an adrenaline orb. In Sun and Moon, this does not apply. Source