That's correct, tavie888. That is the default configuration. That means when a call comes in through the line port (PSTN), it is "routed" to the ph endpoint, i.e. the Phone port.
If you do have a GV number, replacing ph by sp2(GV number) will mean that when a call comes in through the line port, it will be routed via your SP2 (voip.ms) to your Google Voice number.
As you said, you can receive calls just fine via GV, I pressume ending at your Phone Port, i.e. the phone rings, and you can speak and hear correctly. This way, you will receive your PSTN calls through your GV account, but on the same Phone.
Of course this solution is just a patch, and as I said before the 3 services (SP1, SP2 and PSTN) will be busy upon an incoming call from PSTN. I don't know what your problem might be, though. It seems really weird to me.
Another less elegant solution, but easier, would be to put a splitter for the PSTN Line cable, and connect one cable from the splitter to the OBi and another cable to another phone. Of course, you will need the splitter, one more cable, and a spare phone with its own cable.