I've now tested how the app works if I invoke an phone call from phone number specified in Constants.FORBIDDEN_PHONE_CALL_NUMBER and in both Android 8 and Android 10 emulators, this number was intercepted and rejected (it doesn't matter if this phone number was assigned to some person in android phone contacts app).

I'm not sure why it doesn't work for you, but make sure that you write the exact phone number with proper prefix/formatting in Constants.FORBIDDEN_PHONE_CALL_NUMBER.

For example, if you open an emulator window, and in tab 'Phone' you may select the following phone number '+1 650 555-6789', but if you want to intercept this number and reject it, from the app, you must put the following value in Constants:

FORBIDDEN_PHONE_CALL_NUMBER="+16505556789". Note that value is without empty spaces and dashes.

It's a huge difference if you specify FORBIDDEN_PHONE_CALL_NUMBER to be ‘16505556789’ or ‘+16505556789’ (note the ‘+’ character at beginning of second number).