Future<void> _submitPhoneNumber() async {
    /// NOTE: Either append your phone number country code or add in the code itself
    /// Since I'm in India we use "+91 " as prefix `phoneNumber`
    String phoneNumber = "+91 " + _phoneNumberController.text.toString().trim();
    print(phoneNumber);

    /// The below functions are the callbacks, separated so as to make code more redable
    void verificationCompleted(AuthCredential phoneAuthCredential) {
      print('verificationCompleted');
      ...
      this._phoneAuthCredential = phoneAuthCredential;
      print(phoneAuthCredential);
    }

    void verificationFailed(AuthException error) {
      ...
      print(error);
    }

    void codeSent(String verificationId, [int code]) {
      ...
      print('codeSent');
    }

    void codeAutoRetrievalTimeout(String verificationId) {
      ...
      print('codeAutoRetrievalTimeout');
    }

    await FirebaseAuth.instance.verifyPhoneNumber(
      /// Make sure to prefix with your country code
      phoneNumber: phoneNumber,

      /// `seconds` didn't work. The underlying implementation code only reads in `millisenconds`
      timeout: Duration(milliseconds: 10000),

      /// If the SIM (with phoneNumber) is in the current device this function is called.
      /// This function gives `AuthCredential`. Moreover `login` function can be called from this callback
      verificationCompleted: verificationCompleted,

      /// Called when the verification is failed
      verificationFailed: verificationFailed,

      /// This is called after the OTP is sent. Gives a `verificationId` and `code`
      codeSent: codeSent,

      /// After automatic code retrival `tmeout` this function is called
      codeAutoRetrievalTimeout: codeAutoRetrievalTimeout,
    ); // All the callbacks are above
  }

void _submitOTP() {
    /// get the `smsCode` from the user
    String smsCode = _otpController.text.toString().trim();

    /// when used different phoneNumber other than the current (running) device
    /// we need to use OTP to get `phoneAuthCredential` which is inturn used to signIn/login
    this._phoneAuthCredential = PhoneAuthProvider.getCredential(
        verificationId: this._verificationId, smsCode: smsCode);

    _login();
  }

Future<void> _login() async {
    /// This method is used to login the user
    /// `AuthCredential`(`_phoneAuthCredential`) is needed for the signIn method
    /// After the signIn method from `AuthResult` we can get `FirebaserUser`(`_firebaseUser`)
    try {
      await FirebaseAuth.instance
          .signInWithCredential(this._phoneAuthCredential)
          .then((AuthResult authRes) {
        _firebaseUser = authRes.user;
        print(_firebaseUser.toString());
      });
      ...
    } catch (e) {
      ...
      print(e.toString());
    }
  }

  Future<void> _logout() async {
    /// Method to Logout the `FirebaseUser` (`_firebaseUser`)
    try {
      // signout code
      await FirebaseAuth.instance.signOut();
      _firebaseUser = null;
      ...
    } catch (e) {
      ...
      print(e.toString());
    }
  }