PJSIP is open source SIP, Media and NAT traversal library (www.pjsip.org)
This is a prebuilt pjsip library for armv7, armv7s, arm64, i386 and x86_64 architectures.
Compiled with the latest OpenSSL support (by the build script https://github.com/x2on/OpenSSL-for-iPhone)
The best way is to use cocoapods.
Check the Podfile configuration:
platform :ios, '5.0'
pod 'pjsip'
and run pod install