PJSIP is open source SIP, Media and NAT traversal library (www.pjsip.org)
This is a prebuild pjsip library included universal libraries for armv7, armv7s, iPhoneSimulator
The best way is using cocoapods.
Check the Podfile configuration:
platform :ios, '5.0'
pod 'pjsip', '2.1.0'
and run pod install