Kamailio is a SIP server

• Wikipedia says Kamailio, formerly OpenSER, is a GPL-licensed SIP server that can be a registrar, proxy, or redirect server.
• The feature list includes a SIP security firewall, least-cost routing, IMS and VoLTE, a SIP-WebRTC gateway, and MSRP relay.
• The article says a large public deployment, with several million users, is at the German ISP 1&1, and another is at sipgate.
• The documentation page links module docs for series including 6.0.x and 6.1.x, and older series.
• The wiki says its front page links documents for latest stable versions, 5.2 or newer, and points older releases elsewhere.

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