If the OBi is in bridge mode, it acts like a switch. If the OBi were in bridge mode and connected directly to the modem, both it and your pfsense router would receive a different public IP. This would make QoS tricky, but not impossible.
If the OBi were in router mode, and connected directly to your modem, then it could throttle normal internet traffic when a VoIP call was in progress. But since you have a pfsense router, you would probably rather use that for QoS.
