Kamailio (successor of former OpenSER and SER) is an Open Source SIP Server released under GPL, able to handle thousands of call setups per second. It can be used to build large platforms for VoIP and realtime communications like WebRTC, Instant messaging and other applications. Moreover, It can be easily used for scaling up SIP-to-PSTN gateways, PBX systems or media servers like FreeSWITCH, Asterisk or SEMS.
We used Kamailio as load balancer while doing cluster set-up to support high concurrency of call flow. It supports thousand of concurrent calls at meantime to balance the calls alternatively in bunch of servers.
We have comppleted some projects and gain good knowledge about kamailio in terms of integration, using in proxy media and many factors wherein Kamailio used.