Future _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 readable
    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 `milliseconds`
      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
  }