There is no way for a softphone to register directly to the OBi.
I did get CSipSimple to work without registration on WiFi, but others couldn't get it to work on a mobile network. I don't have a data plan for my phone. Even if you could get it to work there is no way to route inbound calls to it outside your LAN. See starting with reply #26 on the same thread.
You could try OBiHai's OBiON app, but people find it buggy with poor quality audio.
It's a much easier install.
http://www.obitalk.com/forum/index.php?topic=47.0