Creates a webhook subscription on your account: a URL you register to be called when a chosen call event happens. A subscription fires on exactly one event — event_trigger must be one of New Call, Ringing, Call Answer, Call Hangup or Fax Content, any other value is stored but never fires — and only a request_type of POST is ever delivered: one HTTPS request to request_url with Content-Type: application/json and a body of exactly {"eventType": "<trigger>"}, with no call ID, phone numbers, timestamp or account in it. There is no signature, delivery ID, retry or delivery log, and custom headers in header_json are stored but not sent — still, set header_json to {} rather than leaving it null, because a null value stops that event from being delivered to any subscription on your account. The new subscription starts enabled whatever you send for enabled, and its ID is not returned: the response body is an empty object, so list your subscriptions to find it.