From Guardian Project Wiki
ostel.me is the official prototype VoIP network for the Guardian Project. It is a work in progress and currently in private alpha testing, therefore a network scan and proof will be forthcoming. It will support all mandatory features for OSTN compliance.
YouTube Video "Getting Started with OSTel Secure Mobile Calling" http://www.youtube.com/watch?v=BrNC8Mk26Z8&feature=youtu.be