# 13807: Client Notification URL Error Response

Log Type: APPLICATION

Log Level: ERROR

## Description

Twilio returns this error when the `clientNotificationUrl` used for a Voice SDK client call notification responds with an error instead of a successful webhook response. Twilio uses this URL to deliver a `POST` request for the Client Call Notification Webhook when you route a call to a Voice SDK client.

### Possible causes

* Your webhook returned a `5xx` response because the application handling the Client Notification URL request failed.
* Your webhook returned `400`, `401`, or `403` because the endpoint rejected the request due to request validation, authentication, authorization, or another server-side configuration issue. Twilio sends this webhook as an HTTP `POST` with a JSON body.
* Your `clientNotificationUrl` points to the wrong endpoint. This commonly results in a `404 Not Found` response.
* Your application returned `404 Not Found` because it could not find a client binding that matched the target identity and Twilio Account SID.
* Your `clientNotificationUrl` is not configured with a valid hostname or includes an underscore in the hostname. For `<Client>` TwiML, this parameter only accepts `POST`.

### Possible solutions

* Verify that `clientNotificationUrl` is set to the correct publicly reachable webhook endpoint in your Calls API request or `<Client>` TwiML.
* Update your webhook handler to accept an HTTP `POST` request with a JSON body and return `200 OK` after you successfully dispatch the notification to the target client.
* If no matching client binding exists, create or refresh the binding before routing calls to that identity. If the user is no longer registered, stop routing calls to that user until a valid binding is available.
* Review any authentication, authorization, or request-validation logic on the endpoint so the webhook request can be processed successfully instead of returning `400`, `401`, or `403`.
* If you handle your own mobile push delivery, pass the notification payload your application receives to `Voice.handleMessage()` on Android or `TwilioVoiceSDK.handleNotification()` on iOS after your application delivers it to the client.

#### Additional resources

* [Client Call Notification Webhook](/docs/voice/sdks/client-call-notification-webhook)
* [TwiML Voice `<Client>`](/docs/voice/twiml/client)
* [Call resource](/docs/voice/api/call-resource)