Phone apps generally provide a pop-up GUI and/or a notification to receive incoming calls. In Kiosk mode, the phone app may not display the pop-up but a user still can take the call through the notification bar.

There are Kiosk mode settings at KM Admin Portal > Profile > Modify Policy > Android Enterprise > Kiosk > Utilities Setting for Recent Apps , System Status Bar , Notification Bar , and Home .

If the Notification Bar is configured to Disallow by default, the user cannot see any incoming calls from the notification bar, even when the phone is ringing. The Notification Bar policy should be set to Allow so that users can drop down the bar and answer the phone.