Flexisip is described as 'SIP proxy server implementation compliant to RFC3261, written in C++11 started in 2011' and is an app in the Social & Communications category. There are four alternatives to Flexisip for Linux, Mac, Windows and BSD. The best alternative is Asterisk, which is both free and Open Source. Other great apps like Flexisip are FreeSWITCH (Free, Open Source), Kamailio (Free, Open Source) and Yate - Yet Another Telephony Engine (Free, Open Source).
FreeSWITCH is a scalable open source cross-platform telephony platform designed to route and interconnect popular communication protocols using audio, video, text or any other form of media. It was created in 2006 to fill the void left by proprietary commercial solutions.