# 16002: Failed to validate conference attributes

Log Level: ERROR

## Description

This error occurs when Twilio cannot validate one or more `<Conference>` attributes on a Voice conference. Check the attribute values, URL formats, and limits against the supported `<Conference>` options.

### Possible causes

* `statusCallbackEvent` includes a value outside `start`, `end`, `join`, `leave`, `mute`, `hold`, `modify`, `speaker`, or `announcement`.
* `recordingStatusCallbackEvent` includes a value outside `in-progress`, `completed`, or `absent`.
* One or more URL attributes, such as `waitUrl`, `statusCallback`, `recordingStatusCallback`, or `eventCallbackUrl`, is missing or is not a relative or absolute URL.
* A method attribute uses a value other than `GET` or `POST`.
* `maxParticipants` is greater than 250, `participantLabel` is longer than 128 characters or is not unique, or `coach` is not a Call SID for a call already connected to the conference.

### Possible solutions

* Update each `<Conference>` attribute to use only supported values and formats. Use boolean values where required, use the documented event names, and keep `maxParticipants` at 250 or below.
* Provide valid relative or absolute URLs for `waitUrl`, `statusCallback`, `recordingStatusCallback`, and `eventCallbackUrl`, and use `GET` or `POST` for `waitMethod`, `statusCallbackMethod`, and `recordingStatusCallbackMethod`.
* Keep `participantLabel` unique and within 128 characters. If you use `coach`, set it to a Call SID that is already active in the conference.
* Prefer `recordingStatusCallback` over `eventCallbackUrl`, because `eventCallbackUrl` is deprecated and Twilio does not send `eventCallbackUrl` requests when `recordingStatusCallback` is provided.

#### Additional resources

* [TwiML Voice: `<Conference>`](/docs/voice/twiml/conference)
* [Voice Conference](/docs/voice/conference)