I know nothing about Talkswitch, but this might be helpful:
The OBi can accept a SIP call and bridge it out to GV, and vice-versa. However, the OBi is not a SIP server, i.e. it will not accept a REGISTER request.
So, if Talkswitch can send a call out on a trunk without registration, just send it to the OBi's IP address and port (default for SP2 is 5061). For example, if Talkswitch can make calls on Voxbeam (a termination provider that does not require or accept a REGISTER request), it should be able to make GV calls through an OBi.
Likewise, if Talkswitch can accept an incoming call on a trunk without registration, just configure SP2 on the OBi to point to the IP address and port of the Talkswitch. For example, if Talkswitch can receive calls from IPKall (an origination provider that does not accept a REGISTER request), it should be able to receive GV calls through an OBi.
For initial testing, just set up SP2 to point to the Talkswitch and tell the OBi to not register. You should be able to call to/from Talkswitch from/to the OBi Phone port. Once that's working, change InboundCallRoute for SP1 from ph to SP2 and InboundCallRoute for SP2 from ph to SP1. Then, test in/out with GV. Of course, if you will also be using the Phone and/or Line ports on the OBi, you will need more complex settings for these parameters.