title: Firebase Authentication with phoneNumber failed with Internal error
body: <p>I want to use <code>Firebase</code> <code>SMS</code> <code>Authentication</code> with phone number.</p>
  
  <p>I have already enabled push notification in my app setting and registered <code>Firebase</code> Apns authentication key as google document says.
  However, after I sent phone number with country code, Xcode says,  </p>
  
  <blockquote>
    <p>Failed to receive remote notification to verify app identity within 5
    second(s)</p>
  </blockquote>
  
  <p>Error says as below:</p>
  
  <pre><code>    error: Error Domain=FIRAuthErrorDomain Code=17999 "An internal error has occurred, print and inspect the error details for more information." UserInfo={error_name=ERROR_INTERNAL_ERROR, NSLocalizedDescription=An internal error has occurred, print and inspect the error details for more information., NSUnderlyingError=0x28329c8a0 {Error Domain=FIRAuthInternalErrorDomain Code=3 "(null)" UserInfo={FIRAuthErrorUserInfoDeserializedResponseKey={
      code = 500;
      errors =     (
                  {
              domain = global;
              message = "Internal error encountered.";
              reason = backendError;
          }
      );
      message = "Internal error encountered.";
      status = INTERNAL;
  }}}}
  
  </code></pre>
  
  <p>My code:</p>
  
  <pre><code>    PhoneAuthProvider.provider().verifyPhoneNumber(phoneNumber, uiDelegate: nil) { verificationId, error in
                  if let error = error {
                      print("error: \(error)")
                      return
                  }
                  print("verificationId: \(verificationId!)")
                  UserDefaults.standard.set(verificationId, forKey: "authVerificationID")
                  self.goToSecondVC()
              }
  </code></pre>
  
  <p>Any idea??</p>