This project was created as a demo using Doubango framework with basic features like making audio/video calls, managing SIP related settings, codec mangement, hardware keyboard support, able to start ringing & accept calls even when the phone is in sleep mode, mute the mic and silence ringtone.
It's been a long time when it was developed. So it may be outdated. But its APK is available at https://docs.google.com/file/d/0B3QXe0IdE5eSVlNlNWdsRjRIYmM/edit?usp=sharing