Add it to Podfile
:
pod "pjsip-ios", git: "https://github.com/Anakros/pjsip-ios.git"
Include needed headers in a Xcode project:
#import <pjnath.h>
#import <pjlib.h>
#import <pjlib-util.h>
You can build it manually from sources, just run pjsip
build-script.