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# detect_fax

> Detect a fax tone on a call.

[detect]: /docs/server-sdks/reference/python/relay/call/detect

Detect a fax tone. A typed convenience over [`detect()`][detect] that builds the
`{"type": "fax", "params": {...}}` configuration for you. All parameters are
keyword-only.

## Parameters

**`tone`** `str | None` — default: None

Which tone to detect: `"CED"` (called station) or `"CNG"` (calling station).
Omit to default to `"CED"`. Keyword-only.

---

**`timeout`** `float | None` — default: None

Maximum seconds to run the detector before stopping. Keyword-only.

---

**`on_completed`** `Callable[[RelayEvent], Any] | None` — default: None

Callback invoked when the operation reaches a terminal state. Can be a regular
function or async coroutine. Keyword-only.

---

## Returns

[`DetectAction`](/docs/server-sdks/reference/python/relay/actions) -- An action handle with `stop()` and `wait()` methods. It resolves on the first detection result.

## Example

```python {12-13}
from signalwire.relay import RelayClient

client = RelayClient(
    project="your-project-id",
    token="your-api-token",
    contexts=["default"],
)

@client.on_call
async def handle_call(call):
    await call.answer()
    action = await call.detect_fax(tone="CED", timeout=20)
    event = await action.wait()

client.run()
```

> Detect a fax tone on a call.