It would appear quite easy for a community to get a powerful alternative to Skype if they have WebRTC in Discourse.
JSCommunicator is a high-level JavaScript API to the WebRTC stack. It includes HTML and CSS fragments that are ready-to-use or can be tweaked to meet Discourse style expectations. It has already been used to create plugins like DruCall for Drupal, the Debian RTC service and the FedRTC service for the Fedora community.
Very little server-side scripting is required: just generate an authentication token or cookie, it can also be configured to use the Discourse password as the SIP password. Most of the real work is done by a SIP proxy and TURN server, they are available as separate packages that sites can install. I recently wrote a step-by-step guide to integrating JSCommunicator in existing open source frameworks like Discourse and I would be happy to provide feedback and access to a pre-configured test instance of the SIP server for anybody who wants to have a go at this.