params = {
  'From' => from,
  'To' => to,
  'EarlyMedia' => true,
  'ConferenceStatusCallbackEvent' => 'start',
  'ConferenceStatusCallback' => Rails.application.routes.url_helpers.call_twilio_conference_callback_url(@call, sid: @call.call_sid, host: DEFAULT_DOMAIN)
}

HTTParty.post("https://api.twilio.com/2010-04-01/Accounts/#{TWILIO['account_sid']}/Conferences/#{@call.browser_call_room_key}/Participants",
               body: URI.encode_www_form(params),
               basic_auth: {username: TWILIO['account_sid'], password: TWILIO['auth_token']})

15003 Call Progress: Error Response to Callback URL