From Guardian Project Wiki
GNU SIP Witch is a call and registration server for the SIP protocol. Because it cannot function as a transparent proxy, it is intended only for user registration and signalling (like ringing a phone) between peers. This creates many networking problems in the most common networking situations. It also reveals the contact IP addresses of both peers in the exchange, which can be used to prove communication across borders. It has a simple design and is much less complicated then any other soft switch we evaluated, but because of the lack of a media proxy, it does not fit in the OSTN spec. There is experimental work to add this capability.
Sip Witch is a young project and good developers. Unfortunately, the documentation of how to configure the core to allow two users to call each other is absent. This work is in progress and will improve with time. There is a Chef cookbook to automate a server deployment.
The documentation wiki has an annotated XML file that you can glean some bits to copy and paste for your particular configuration.