Kamailio® is an open-source SIP server, which evolved from the earlier OpenSER and SER, and is distributed under GPLv2+, capable of managing thousands of call setups each second. This robust platform can be utilized for constructing extensive systems for VoIP, real-time communication, presence, WebRTC, instant messaging, and various other applications. Additionally, Kamailio is well-suited for scaling SIP-to-PSTN gateways, PBX systems, or media servers such as Asterisk™, FreeSWITCH™, or SEMS with ease. Its extensive feature set includes support for asynchronous TCP, UDP, and SCTP, secure communication through TLS for voice, video, and text, as well as WebSocket compatibility for WebRTC. The server supports both IPv4 and IPv6, offers simple instant messaging and presence capabilities with an integrated XCAP server and MSRP relay, and enables asynchronous operations. Furthermore, it includes IMS extensions for VoLTE, ENUM functionality, DID and least cost routing, load balancing, and routing failover. Kamailio also provides comprehensive accounting, authentication, and authorization features, while supporting a variety of backend systems, including MySQL, Postgres, Oracle, Radius, LDAP, Redis, Cassandra, MongoDB, and Memcached. All these capabilities make Kamailio a versatile choice for modern communication infrastructures.