You should be on Firebase's Blaze Plan in order for Phone authentication to work. You can find more info about the pricing here
Real Phone numbers will not work on any Emulator,it's Firebase limitation, if you want to test it on emulators you have to Add Testing Numbers
if you face a problem when verifying Phone Number 'unknown error occured' make sure that you have added your SHA1 Fingerprint here
you may see some 'warnings' while building your app on Android Studio, please ignore them as they are just warnings and NOT 'errors' , so they have no effect on your app.
We use DI(Dependency Injection) to make our code cleaner, So If you passed any new class into a constructor or changed the constructor, you have to clean the project, otherwised it will show a build-time error. To do that go to Android Studio > Build > Clean Project.