If you want to develop a ios voip softphone from scatch, you can use this project.
It has a sip library written in objective c, where registration mesagges are implemented.
Feel free to extend this library.
First clone this repository then set your sip server ip, port, username and password in "SipServerInfo.plist" file.
Framework: MVC Architechture.
Protocol: SIP(RFC 3261) , UDP
Encryption Digest: MD5
Language: Objective c.
Usually, you will find lots of open source SIP libraries such as pjsip, linphone, jsSip etc. But there architechture is too much complex
for initial or basic development. In future, I will include audio codec such as G729 & G711 and RTP in this project.
If you dont know the basics of SIP protocol, i suggest you go to this link.
Sip Basics
For more information, please go through the project and feel free to create an issue for any misunderstading.