In order to receive incoming calls via SIP URI, you must have at least one service (SP1 and/or SP2) configured as SIP. It need not have an actual SIP provider operational on it, but by being configured as SIP, it cannot be used for Google Voice.
Your unused SP2 can be configured for SIP as follows:
Service Providers -> ITSP Profile B -> SIP -> ProxyServer : 127.0.0.1
Voice Services -> SP2 Service -> AuthUserName : (put anything here)
Voice Services -> SP2 Service -> X_RegisterEnable : (unchecked)
Voice Services -> SP2 Service -> X_ServProvProfile : B
SP2 status should show : Registration Not Required
You will also need to forward port 5061 to your OBi.
SIP URI's can then be addressed to : anything@obi100_public_IP:5061