Each Gym Leader is away from their Gym at a certain time of the Day.
Each Gym Leader has a certain day in which if you call them they will Show up in the Fighting Dojo
You need to find them and register their Poke-Gear Number. The Gym Leadera are all scattered around in the Kanto Region.
Here is a list of where you can find them
There are a few exceptions :
Talk to his wife outside his Gym to get the PokéGear number.
After you catch Zapdos he will appear in front of the Power Plant.
Show him a Pikachu and he will register you
Talk to his sister in Pallet Town and get 5 massages from her.
After show her a Pokemon with Max Happiness and she will hand over Blue number e_e
This will help you