title: Flutter + Firebase - Link Phone Number with Existing Account
body: <p>I have implemented Email Authentication using Firebase in Flutter. I want to take the phone number from the user and verify it by sending an OTP to ensure it belongs to them and then link their phone number with the account they created earlier using email.</p>
  <p>For this purpose I did the following:</p>
  <pre><code>verifyPhoneNumber() async {
      await auth.verifyPhoneNumber(
        phoneNumber: '+92${widget.phoneNumber}',
        verificationCompleted: (PhoneAuthCredential credential) async {
          await auth.currentUser.updatePhoneNumber(credential);
          // either this occurs or the user needs to manually enter the SMS code
        },
        verificationFailed: (FirebaseAuthException e) {
          print(e.message);
        },
        codeSent: (String verificationId, int resendToken) {
          setState(() {
            _otpcode = verificationId;
          });
        },
        codeAutoRetrievalTimeout: (String verificationId) {
          setState(() {
            _otpcode = verificationId;
          });
        },
      );
    }
  </code></pre>
  <p>The above code sends the OTP to the number. Now when the user enters the received code into the TextField I want to verify that both match so I can proceed with updatePhoneNumber.</p>
  <p>I am not sure how to implement the verification part. I have the following code and not sure how to output the errors if any or to confirm the verification did happen.</p>
  <pre><code>verifyPhoneCode(otp, code) async {
    final AuthCredential credential =
        PhoneAuthProvider.credential(verificationId: otp, smsCode: code);
    await auth.currentUser.updatePhoneNumber(credential);
  }
  </code></pre>