The stock native android sip stack is built on top of JAIN SIP but it does not have all functionality, there is no support for IM, presence and video calls right now. Google forked SIP Stack and never really updated the SIP API that is shipped by default and used the same package names as the original JAIN SIP project. This proved to be a major hassle to developers that wanted to add SIP capabilities worldwide as they couldn’t use JAIN SIP out of the box.
An opensource implementation of the Android JAIN SIP stack can be found here:
https://code.google.com/p/jain-sip/
It is a java based SIP stack which allows you to integrate the sip stack seamlessly in your android application without the hassle of NDK.
A Complete SIP Stack
JAIN SIP is a full implementation of the RFC 3261 Specification and as well as support for several SIP RFCs.
What is the issue using the existing JAIN SIP stack?
If you have tried to use the existing JAIN SIP in any android application you might have run into package name conflicts, these conflicts are resolved now and the stack is ready to be used in android.
The full blog post with example code and Android APK is available here:
http://www.telestax.com/jain-sip-stack-for-android/