A reddit user FrozenPenguinToaster posted video about the Pokemon incoming call bofore, now Pokecall is finally released. It’s functional, with contacts showing up as trainers, and presenting the options to answer the call (fight) or hang up (run away). 

Compatibility:  Supports iOS 9.0 - iOS 11.1.2
Please note: Devices on iOS 10 appear to be having issues and will be fixed ASAP.

Features:

• Pokémon Gold/Silver/Crystal battle interface

• Choose your Pokémon from any in the entire Johto and Kanto regions (1st and 2nd generation)

• Have trainer battles or wild Pokémon encouters

• Transform your ringtone into one of the many different battle theme songs

• Option to minimise the call screen to a banner

• Applies to FaceTime calls and third party calling apps such as WhatsApp

After you enble this tweak in Settings, you also need to choose the Pokémon.

Repo: https://repo.packix.com/package/com.fpt.pokecall/