1. You might try forwarding to a free VoxOX number, which in turn forwards to your destination. In VoXox Preferences -> Phone Settings -> Ringback & Hold Music, you can record or upload whatever you'd like. Unfortunately, such an approach might reduce the quality of your call, add latency and/or degrade reliability.
2. Try changing Tone Settings -> Tone Profile A -> Ringback Tone -> TonePattern. For example, the setting for a UK ring is 400-18,425-18,450-18;-1;(.4+.2,.4+2) . This approach will also affect the ringback you hear on outbound calls placed from your OBi.