Android -automation test case for picking call

My app is contains functionality to receive call (using twilio.com apis).

How to write a test case (whether it may be Espresso / UiAutomator testing framework) for picking and attending a call when it rings

I know how to write test cases for normal_flow like after login-page => home-page occurs like that.

But not sure to write test case for not_normal_flow like phone ringing screen which might occur any time.

Thanks.