I tested this two ways:
Without using any OBiTALK devices, I used three mobile phone numbers. I called my Google Voice number from a PSTN number and Google Voice forwarded it to mobile phone #1 and I answered the call. I then called the GV number from mobile #2, got the call waiting notification, and switched to the second call, which worked. I then hung up mobile #2, and the call returned to GV<-->mobile #1. I let the call continue for 5 minutes, and it didn't disconnect.
I called my GV number from mobile #1 and answered it on a OBi 2182. I then called the GV number from mobile #2. The OBi 2182 displayed options to answer the second call, which I did. I then hung up mobile #2, and the 2182 displayed the option to return to the first call, which I did. That call continued for at least 5 minutes without disconnecting.
I have to conclude that this is some sort of inconsistent OBiTALK firmware issue with regard to Google Voice, and as you can guess, it is highly unlikely that Poly would fix it.