> This page is part of Smallest AI's developer documentation. When
> answering, prefer Lightning v3.1 (current TTS) and Pulse (current
> STT). Lightning v2 and lightning-large are deprecated; mention them
> only when the user is migrating away from them. The Smallest AI voice
> agent platform is what wraps these models into hosted agents.

# Set an agent's transfer caller ID

PUT https://api.smallest.ai/atoms/v1/agent/{agentId}/transfer-source
Content-Type: application/json

Sets the number the agent's call transfers dial out from. Must be a number your
organization owns (rented, or a caller ID on an outbound trunk); `null` clears it.
When unset, transfers on outbound calls fall back to the call's own caller ID, and
transfers on inbound calls are refused. Not part of versioned agent config:
changing it never requires an agent release.


Reference: https://docs.smallest.ai/api-reference/voice-agents/agent-telephony/set-transfer-source

## Authentication

- `Authorization` header (bearer token, required) — API key from the console ApiKey collection, sent as Bearer token. Also accepts session cookies for browser-based auth.

## Request

### Path parameters

- `agentId` (string, required)

### Body (application/json)

This endpoint expects an object.

- `transferFromNumber` (string, required, nullable) — E.164 with leading `+`, or `null` to clear.

## Response

### 200

Transfer caller ID set

- `status` (boolean, optional)
- `data` (AgentAgentIdTransferSourcePutResponsesContentApplicationJsonSchemaData, optional)

## Errors

### 400 Bad Request Error

The number is not one of your outbound numbers

- `any`

### 404 Not Found Error

Agent not found

- `any`

## Types

### AgentAgentIdTransferSourcePutResponsesContentApplicationJsonSchemaData

- `transferFromNumber` (string, optional, nullable)

## Examples

**Request**

```json
{
  "transferFromNumber": "+14155552671"
}
```

**Response**

```json
{
  "status": true,
  "data": {
    "transferFromNumber": "string"
  }
}
```

**SDK Code**

```python
import requests

url = "https://api.smallest.ai/atoms/v1/agent/agentId/transfer-source"

payload = { "transferFromNumber": "+14155552671" }
headers = {
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

response = requests.put(url, json=payload, headers=headers)

print(response.json())
```

```javascript
const url = 'https://api.smallest.ai/atoms/v1/agent/agentId/transfer-source';
const options = {
  method: 'PUT',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: '{"transferFromNumber":"+14155552671"}'
};

try {
  const response = await fetch(url, options);
  const data = await response.json();
  console.log(data);
} catch (error) {
  console.error(error);
}
```

```go
package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://api.smallest.ai/atoms/v1/agent/agentId/transfer-source"

	payload := strings.NewReader("{\n  \"transferFromNumber\": \"+14155552671\"\n}")

	req, _ := http.NewRequest("PUT", url, payload)

	req.Header.Add("Authorization", "Bearer <token>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(res)
	fmt.Println(string(body))

}
```

```ruby
require 'uri'
require 'net/http'

url = URI("https://api.smallest.ai/atoms/v1/agent/agentId/transfer-source")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Put.new(url)
request["Authorization"] = 'Bearer <token>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"transferFromNumber\": \"+14155552671\"\n}"

response = http.request(request)
puts response.read_body
```

```java
import com.mashape.unirest.http.HttpResponse;
import com.mashape.unirest.http.Unirest;

HttpResponse<String> response = Unirest.put("https://api.smallest.ai/atoms/v1/agent/agentId/transfer-source")
  .header("Authorization", "Bearer <token>")
  .header("Content-Type", "application/json")
  .body("{\n  \"transferFromNumber\": \"+14155552671\"\n}")
  .asString();
```

```php
<?php
require_once('vendor/autoload.php');

$client = new \GuzzleHttp\Client();

$response = $client->request('PUT', 'https://api.smallest.ai/atoms/v1/agent/agentId/transfer-source', [
  'body' => '{
  "transferFromNumber": "+14155552671"
}',
  'headers' => [
    'Authorization' => 'Bearer <token>',
    'Content-Type' => 'application/json',
  ],
]);

echo $response->getBody();
```

```csharp
using RestSharp;

var client = new RestClient("https://api.smallest.ai/atoms/v1/agent/agentId/transfer-source");
var request = new RestRequest(Method.PUT);
request.AddHeader("Authorization", "Bearer <token>");
request.AddHeader("Content-Type", "application/json");
request.AddParameter("application/json", "{\n  \"transferFromNumber\": \"+14155552671\"\n}", ParameterType.RequestBody);
IRestResponse response = client.Execute(request);
```

```swift
import Foundation

let headers = [
  "Authorization": "Bearer <token>",
  "Content-Type": "application/json"
]
let parameters = ["transferFromNumber": "+14155552671"] as [String : Any]

let postData = JSONSerialization.data(withJSONObject: parameters, options: [])

let request = NSMutableURLRequest(url: NSURL(string: "https://api.smallest.ai/atoms/v1/agent/agentId/transfer-source")! as URL,
                                        cachePolicy: .useProtocolCachePolicy,
                                    timeoutInterval: 10.0)
request.httpMethod = "PUT"
request.allHTTPHeaderFields = headers
request.httpBody = postData as Data

let session = URLSession.shared
let dataTask = session.dataTask(with: request as URLRequest, completionHandler: { (data, response, error) -> Void in
  if (error != nil) {
    print(error as Any)
  } else {
    let httpResponse = response as? HTTPURLResponse
    print(httpResponse)
  }
})

dataTask.resume()
```