That suggests to me that you've created a calling loop, such that two inbound calls are simultaneously ringing your OBi, creating a call waiting condition during your call. After you hang up, the waiting call rings your OBi, until it recognizes that the other party hung up (from your actual call).
You didn't provide any details about how your OBi is configured (which service provider(s) are you using, and if it's Google Voice, how you are using GV -- either directly, or having inbound calls forward to a SIP ITSP).