Definition

In CallKit, apps can interact with two different types of interfaces:

• System Call UI → the native iOS calling interface managed by CallKit
• Custom UI → app-created interfaces built using UIKit or SwiftUI

Most VoIP apps use a combination of both.

Core Concept

CallKit allows communication apps to integrate with the native iOS telephony experience while still optionally maintaining custom in-app call screens.

This creates two UI layers:

System UI
managed by iOS

Custom UI
managed by your app

System Call UI

The system call UI is:

the native iOS call interface controlled by CallKit.

Examples:

• lock screen incoming call screen
• call banners
• Dynamic Island call state
• native in-call controls