kamailio/kamailio

Kamailio (formerly OpenSER) is a foundational piece of the world's telecommunications infrastructure. With over 12 years of history and a massive contributor base (indicated by 1,000+ forks), it is the de facto standard for high-performance SIP routing. Its defensibility is rooted in the extreme complexity of the SIP protocol and the decade-plus of edge-case handling for interoperability with thousands of different hardware and software vendors—a feat that is nearly impossible to replicate quickly even with modern AI. While its competitor, OpenSIPS (a fork of the same project), provides a similar capability, the two projects largely dominate the open-source market for SIP proxies. Frontier labs (OpenAI/Google) are highly unlikely to compete here as SIP is a legacy/infrastructure-level protocol that serves as a plumbing layer for the voice interfaces they might build; they would likely use Kamailio or a managed service built on it rather than reinvent it. The project's 'moat' is essentially the collective knowledge of the VoIP industry embedded in its C modules. Platform domination risk is low because telecommunications providers often require the level of control and regulatory compliance that only a self-hosted, highly configurable engine like Kamailio provides.