Are there any non-GPL SIP libraries/SDKs that'll let me implement SIP for iOS?
alinoz
oSIP is LGPL and therefore can be used (as a library) in commercial and closed-source projects. It compiles easily for arm. It is the SIP library used by linphone for iPhone; however, linphone is GPL.
These days SIP over WebRTC is getting popularity. It uses SIP Signaling and WebRTC as Media which is provided by google.
Following options are also very good if you to avoid getting into SIP level complexities.
来源：https://stackoverflow.com/questions/10854442/sip-library-for-ios-with-non-gpl-license