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pjsua is an open source command line SIP user agent (softphone) that is used as the reference implementation for PJSIP, PJNATH, and PJMEDIA. Despite its simple command line appearance, it does pack many features!
This library use for SIP handler. More information about this library you will found in this site http://www.pjsip.org/pjsua.htm
How to use it and how to install will explain here: