is the system-level user interface component in the Android operating system that handles the on-screen display and controls during an active phone call.

In short:

When you make or receive a call, the screen you see — with the caller ID, answer/decline buttons, mute, speakerphone, keypad, and end-call options — is generated by InCallUI. It’s part of the Dialer app’s underlying code but can be updated separately via system updates or vendor customizations (e.g., Samsung’s One UI, Google Pixel’s Phone app).