> ## Documentation Index
> Fetch the complete documentation index at: https://www.voiceflow.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Voice & telephony

> Trace definitions, payloads, and handling guidance for voice & telephony.

[All traces](/docs/api-reference/trace-types)

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## `audio`

An audio response event. The payload's `state` field selects the shape: `start` announces a `messageID`, `content` carries the audio in `content` with its `encoding`, and `end` carries the state alone.

| Field | Type | Required | Present when |
| --- | --- | --- | --- |
| `content` | string | no | `state` is `content` |
| `delay` | number | no | `state` is `start` |
| `duration` | number | no | `state` is `content` |
| `encoding` | `"audio/mp3"` or `"audio/x-mulaw"` or `"audio/pcm"` | no | `state` is `content` |
| `messageID` | string | no | `state` is `start` |
| `state` | `"start"` or `"content"` or `"end"` | yes | always |

## `speak`

A spoken response for voice channels, carrying the message text and optionally a synthesized audio source and voice name.

| Field | Type | Required |
| --- | --- | --- |
| `ai` | boolean | no |
| `isPrompt` | boolean | no |
| `message` | string | yes |
| `src` | string | no |
| `type` | `"audio"` or `"message"` | yes |
| `voice` | string | no |

## `stream`

An audio stream event. The payload carries the source in `src`, a `token`, and an `action` of `LOOP`, `PLAY`, `PAUSE`, or `END`; `title`, `description`, `iconImage`, `backgroundImage`, and `loop` are optional.

| Field | Type | Required |
| --- | --- | --- |
| `action` | `"LOOP"` or `"PLAY"` or `"PAUSE"` or `"END"` | yes |
| `backgroundImage` | string | no |
| `description` | string | no |
| `iconImage` | string | no |
| `loop` | boolean | no |
| `src` | string | yes |
| `title` | string | no |
| `token` | string | yes |

## `dtmf`

Configures keypad (DTMF) input collection on a call: whether it is enabled, the timeout, the ending delimiter, and the digit cap.

| Field | Type | Required |
| --- | --- | --- |
| `delimiter` | `"#"` or `"*"`[] | yes |
| `enabled` | boolean | yes |
| `maxDigits` | number | yes |
| `timeoutInSeconds` | number | yes |

## `call-forward`

Signals that the call is being forwarded. The payload always carries the destination in `address` and its kind in `addressType`; a `phone` forward may also carry an `extension` and a `callerIDPassthrough` flag. See the call forward step for configuration.

| Field | Type | Required | Present when |
| --- | --- | --- | --- |
| `address` | string | yes | always |
| `addressType` | `"phone"` or `"sip"` | yes | always |
| `callerIDPassthrough` | boolean | no | `addressType` is `phone` |
| `extension` | string | no | `addressType` is `phone` |
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