The Android SDK is developed in parallel with the Java SE SDK and our Android softphone.
The reason why we built a SIP SDK for the Android platform was driven by the needs to release a performant and optimized softphone for this platform.
After looking for the alternatives, including open-source and commercial solutions, we have found that there is no any SIP stack targeting specifically the Android platform, except the built-in SIP API which is a dumb implementation, not suitable for production usage.
We found that all third party implementations are ported from other platform/language (usually from legacy C/C++) which often results in a bloated library, difficult or impossible to optimize for this platform.
Fortunately we already had a rock solid pure Java implementation for VoIP which was easy to extend with the Android specific code (user interface, audio device handling, battery saver, push notifications and others).
With other words, our Android SIP SDK is a result of our Java SE library (first release in 2008) and our Android softphone (first release in 2011) with most of their code-base shared and we take special care to highly optimize for the Android platform with continuous integration tests.
All these three software already reached a mature state, suitable for commercial usage and all software components are in active continuous development.
We release a new major version for our Android SDK in around every year (this is also published on our website), however internally we release a new stable version at almost every week, so our customers always have access to the latest new version.
We don't maintain a separate version history for the SDK since this is a combination of the Softphone and the Java lib for which you can find the version history here and here (just merge the two logs to find out what is new in the AJVoIP SDK).