Flexisip Server
Flexisip is a complete, modular and scalable SIP server suite that includes proxy, presence, and group chat functions.
Flexisip offers an easy-to-install SIP server solution, offering all the features required to deploy your own SIP service tuned for mobile or desktop applications, « out of the box ».
The free sip.linphone.org SIP service has run on Flexisip since 2011, and enables Linphone users to create their own SIP addresses to connect with each other.