I have this page in my browser as an interesting project, called VillageTelco, developed by a group of people for the 'more remote places' of this planet. The have developed low power gear and used open source tools to build "easy-to-use, scalable, wireless local, DIY, telephone company toolkit". It works on a mesh principle, and they call their devices "Mesh Potatoes".
When looking at kernel commits, I saw BATMAN, and was wondering what it was. That led to open-mesh, which lead to their projects page, which linked to the VillageTelco project.
Related to IP Telephony, in an open-source world, I also came across Kamailio:(successor of former OpenSER and SER) is an Open Source SIP Server ... used to build large platforms for VoIP and realtime communications – presence, WebRTC, Instant messaging and other applications. On their web site:
Among the powerful features: asynchronous TCP, UDP and SCTP, secure communication via TLS for VoIP (voice, video, text); WebSocket support for WebRTC; IPv4 and IPv6; SIMPLE instant messaging and presence with embedded XCAP server and MSRP relay; asynchronous operations; IMS extensions for VoLTE; ENUM; DID and least cost routing; load balancing; routing fail-over; accounting, authentication and authorization; support for many backend systems such as MySQL, Postgres, Oracle, Radius, LDAP, Redis, Cassandra, MongoDB, Memcached; Json and XMLRPC control interface, SNMP monitoring.
A related SIP site is intel.org, which is home to the SIP Express Router, SIP Express Media Server, and SIP Express Router Web.
CSRP is a captive solution making use of the above products. I can't vouch for the product, just saw it in passing, which lead to the above resources.