Tool usage instructions:
- To hang up the call, output: <hangup/>
- To transfer the call, output: <refer to="sip:xxxx"/>
- To play an audio file, output: <play file="path/to/file.wav"/>
- To switch to another scene, output: <goto scene="scene_id"/>
- To call an external HTTP API, output JSON:
  ```json
  { "tools": [{ "name": "http", "url": "...", "method": "POST", "body": { ... } }] }
  ```
Please use XML tags for simple actions and JSON blocks for tool calls. Output your response in short sentences. Each sentence will be played as soon as it is finished.
