The schema is of type enum<string>.

Available options:
registered,
queued,
dispatching,
provider_queued,
initiated,
ringing,
in-progress,
user-ended,
agent-ended,
api-ended,
voicemail-hangup,
voicemail-message,
timeout,
canceled,
busy,
no-answer,
failed,
error,
unknown,
chat_completion