<a href="https://en.wikipedia.org/wiki/CSipSimple" rel="nofollow">https://en.wikipedia.org/wiki/CSipSimple</a> (Android SIP client)<p>For a more low-level stuff, it might worth having a look at <a href="http://www.pjsip.org/" rel="nofollow">http://www.pjsip.org/</a> (a really good SIP library written in C with among others, Java bindings; CSipSimple is based on it).