PJSIP is a free and open source multimedia communication library
written in the C language implementing standard based protocols such as
SIP, SDP, RTP, STUN, TURN, and ICE.  It combines signaling protocol
(SIP) with rich multimedia framework and NAT traversal functionality
into a high level API that is portable and suitable for almost any type
of systems ranging from desktops, embedded systems, to mobile handsets.

This library is required by Asterisk if you wish to use the newer PJSIP
communication features included with Asterisk 12 and above.

Optional dependencies: ffmpeg