8 Open source SIP Servers
OpenSIPS is a free open source SIP proxy/ server that supports voice, video, IM, presence, and other SIP extensions.
OpenSIPS team offers a LTS support for latest stable release, and it is available for Linux servers (Ubuntu, Debian, Fedora, openSUSE, RedHat, and CentOS).
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Hermes is a modern SIP server framework for building real-time SIP apps. Hermes will substitute old legacy SipServlet. It is based on reactive manifesto.
Hermes is meant for Java developers, and it is a FLOSS (Free Libre Open Source Software) under the GNU Lesser General Public License.