The plugin loads url from 'plugin.callback_on_offline.url' property, adds 'to' & 'from' parameters and executes GET request. The request is async. Sample link: http://localhost:8080/user/offline/callback/url?from=1%40fotsum.com&to=2%40fotsum.com
If something don't work, set "plugin.callback_on_offline.debug" property to true and check the debug log.