An OBi is not a SIP server -- it will not respond to a REGISTER request. On the PAP2, you can set Register to no and Make Call Without Reg and Answer Call Without Reg to yes.
Assuming that SP2 is configured for SIP:
For incoming, set InboundCallRoute for the Line port to e.g. {ph,sp2(1234@192.168.1.123:5064)} where 1234 is the User ID on the PAP2, 192.168.1.123 is the PAP2's IP address, and 5064 is the SIP Port for the PAP2 line in question. To get caller ID, you'll need to set spoofing for SP2 (with appropriate measures if the provider on SP2 does not support spoofing).
For outgoing, the PAP2 sends the call (via Proxy, the dialplan or a speed dial) to 192.168.1.234:5061 where 192.168.1.234 is the IP address of the OBi and 5061 is the value of X_UserAgentPort for SP2. InboundCallRoute for SP2 needs to distinguish a PAP2 call from a service provider call and route it to the Line port. If your router exposes the port to the Internet, this is a big security risk. You might filter based on the caller ID from the device and/or a "secret" prefix added to the destination number. You might also restrict such calling to local or domestic numbers.
The Gigaset does not provide an option for not registering and will continuously show registration failure. However, it can still make and receive calls on that provider slot.
An alternative to all these constraints is to put e.g. a PBXes account in the path. If you set up the OBi as a "sub PBX", you can still have single stage dialing to the Line port. I took this approach, in part because it gives me free voicemail for the POTS line.
