Boomshnka, this thread is old, and I am no longer in possession of the OBi200 I set this up on. However, I just spoke with the original party and I *believe* this is how I did it: My friend had claimed a GV number to experiment with before porting her home and cell numbers to GV and FI respectively. She created separate accounts for these ports. This left her with two GV numbers (the original, plus her ported number) plus the FI number. Let's say the original GV number is on SP1 and the ported home phone number is on SP2. When a call comes in on SP2, I had it set up to ring the phone attached to the OBi box and simultaneously forward the call to her FI number using the SP1 trunk. It worked, but the latency on a FI connection was quite noticeable due to the delays in the call transiting the internet twice. The FI phone also always showed her GV number as the callerid instead of the real id of the calling party.
I believe your problem is that, although the OBi does support 3-way calling, it allows only one call to be set up on a given SPx at a time. (This may be a GV constraint, but I think it is the OBis.) In any event, your method won't work because you cannot originate the outgoing call until the incoming call is answered. Catch-22. That's why I had to use the original GV number to forward the call. (At least that's how I remember it -- I'm sure someone will correct me if I am wrong.)
Hope this helps. Good luck!