If the Polycom can place calls without registration, you can point the phone's Proxy parm at the OBi202's SP4 address and port plus adding some rules to the InboundCallRoute.
If it can't call without registration, you need to redefine SP4 as a Proxy and register the phone to it.
You can then define a Voice Gateway on the OBi202 for outbound Callcentric calls.
Inbound Callcentric calls can be routed to your public IP address. You will need to setup port forwarding in your router.
What trunks do you want to use on the Polycom? What trunk do you want as the default?
Here are instructions I previously wrote:
OBi202:
Service Providers -> ITSP Profile D SIP -> ProxyServer: 127.0.0.1
Service Providers -> ITSP Profile D SIP -> X_SpoofCallerID: Checked
Voice Services -> SP4 Service -> AuthUserName: OBi202
Voice Services -> SP4 Service -> AuthPassword: OBi202PW
Voice Services -> SP4 Service -> X_RegisterEnable: Unchecked
Voice Services -> SP4 Service -> X_ServProvProfile: D
Voice Services -> SP4 Service -> X_Proxy: Checked
Voice Services -> SP4 Service -> X_InboundCallRoute: {OBi202>(Msp1):sp1}
Voice Services -> SP1 Service -> X_InboundCallRoute:
ph,SP4(OBi202@local_client)
IP Phones:
Proxy: 192.168.1.100 (IP address of the OBi202)
Proxy Port: 5063 (X_UserAgentPort of OBi202 SP4 trunk)
UserID: OBi202
Password: OBi202PW