WebSock transport for an authenticated and authorized Twilio call.

The host supplies start_session: fn channel -> ... end, setting μ-law as the session's audio_format, and event_sink: self(). Supply authorized account/call IDs in channel_options. Authenticate before upgrading. on_ready may send a greeting. The host owns admission and call duration. Socket closure tears down the session; it does not issue a Twilio hangup API.