PJSIP library is an well known open-source library developed by Teluu under GPL or commercial license and implementing the Session Initiation Protocol (SIP) stack. It is written in C and it is available on all major operating systems (e.g. macOS, Linux, Windows, Android, iOS). Its primary use is in VoIP applications based on SIP and Real Time Protocol (RTP). PJSIP library is also one of the SIP stacks used by Asterisk communications framework.