# POST /v1/convai/twilio/register-call (ElevenLabs API)
Base URL: https://api.elevenlabs.io
Auth: xi-api-key: <ELEVENLABS_API_KEY>
Last verified: 2026-09-03

> Register A Twilio Call And Return Twiml

## Critical Gotchas
* Authentication uses the non-standard `xi-api-key` header, not `Authorization: Bearer`. Sending a Bearer token returns HTTP 401.
* Text-to-speech responses are raw binary audio, not JSON. Write the body to a file (`--output speech.mp3`) rather than parsing it.

## cURL
curl -X POST 'https://api.elevenlabs.io/v1/convai/twilio/register-call' \
  -H "xi-api-key: $ELEVENLABS_API_KEY" \
  -H 'Content-Type: application/json' \
  -d '{"agent_id":"string","from_number":"string"}'

## Parameters
| Name | In | Type | Required | Description |
| :--- | :--- | :--- | :--- | :--- |
| agent_id | body | string | Yes | Request body field agent_id. |
| from_number | body | string | Yes | Request body field from_number. |
| to_number | body | string | Yes | Request body field to_number. |
| direction | body | string | No | Request body field direction. |

## Response (200 OK)
"string"

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