Much easier solution, assuming all you care about is using Vopium. Just because your Vopium account is on SP2 doesn't mean it has any visibility to your GV number. When you call via the OBi, you're using your Vopium log in info somehow.
Set your office number as one of the Google Voice "Phones" in the settings of GV.
When at your office, open up GV and make a call via your Vopium account, telling GV to call your office phone under "Phone to call with". GV will ring your office phone, when you pick up it will call the number that you gave it for your Vopium account (I'm assume this is a phone number you call and then you do something to dial an international number).
If not, perhaps something more sophisticated is needed.