- [Kamailio](http://www.kamailio.org/w/) - Modular SIP server (registrar/proxy/router/etc). ([Source Code](https://github.com/kamailio/kamailio)) `GPL-2.0``C`
- [Kazoo](http://2600hz.org/) - KAZOO is an open-source, highly scalable software platform designed to provide carrier-grade VoIP switch functions and features. ([Source Code](https://github.com/2600hz/KAZOO)) `MPL-1.1``Erlang`
- [Tapir](http://www.sip3.io/) - Troubleshooting and real-time monitoring of VoIP-based systems. ([Source Code](https://github.com/sip3io/tapir)) `Apache-2.0``Java/Kotlin`
- [Wazo](http://wazo.community/) - Full-featured IPBX solution built atop Asterisk with integrated Web administration interface and REST-ful API. ([Source Code](https://github.com/wazo-pbx)) `GPL-3.0``Python/PHP`