I have used the OBi with Polycom phones in the manner described and had no trouble. You need to see what's going wrong. For starters, what does Call History show (there may be a SIP error code, etc.)? If no luck, try SIP Debug, sending the output to your PC (use Wireshark or a syslog server to view). Or, use SIP Debug on the Cisco.
An alternate approach is to register SP2 to your PBXes account as a sub-PBX. With the Cisco registered as an extension, you can set it up for single-stage dialing on outgoing, and get caller ID to pass on incoming. My OBi is presently set up that way (I use IP phones exclusively and have nothing connected to the Phone port).