It is likely the ObiApp or OBiON isn't turned on (logged into OBiTALK). It is like "presence" status.
If you're certain that your OBiApp is actually logged in, and yet the status still shows yellow, it might be that it did not get updated yet within the registration interval (doesn't mean you can't make/receive calls from the softphone).
We have noticed that occasionally the status turns yellow yet the device is fully logged-in.