How to unit test callback based method (Firebase's verifyPhoneNumber(...))

I'm trying to unit test a my AuthenticationService class, which acts as an abstraction of Flutter's plugin firebase_auth. Specifically, I want to test following method:

Future<void> startAuthFlow(String phoneNumber) async {
  await _firebaseAuth.verifyPhoneNumber(
    phoneNumber: phoneNumber,
    verificationCompleted: _onVerificationCompleted,
    verificationFailed: _onVerificationFailed,
    codeSent: _onCodeSent,
    codeAutoRetrievalTimeout: _onCodeAutoRetrievalTimeout,
  );
}

I'm not sure about how to approach the unit testing, as I would like to assure that after SMS is sent, verification is completed, etc, the logic in the private callbacks is executed. However, I don't know how to mock such a function.

I've also thought about the possibility of just unit-testing the callbacks independently, and considering their calling by _firebaseAuth.verifyPhoneNumber as subject of an integration test. However, as those callbacks are private, there is no a good way to test them.