The "soft phone" is incredibly poorly documented.
It is a OBi number assigned for your use (if you wish) with OBi's "Obion" VoIP client software. It can't be deleted off of the dashboard, so you can just ignore it. They had Obion apps for iPhone, Android and PC. They work poorly, if at all, and are in need of maintenance. Their main benefit was to call into your OBi over VoIP from your smartphone, and access Your OBi's "auto-attendant", which would then allow you to bridge out to a new call, or call some other OBi.
If you are just interested in using your OBi with Google Voice, the Obion apps aren't necessary, since you can also make and receive calls using your Google Voice phone number, by installing the Google Hangouts apps for iOS or Android, or using the Hangouts web app with the Google Chrome Browser.