You will usually get better quality and lower latency if you make the call via the GV auto atttendant, rather than going through the OBi.
With default settings on GV (and the OBi not answering the call), you would wait for GV voicemail to answer, press * during the greeting, enter your GV PIN, then press 2 to make an outgoing call.
You can set up GV so calling from your office number takes you directly to the GV auto attendant; you would then just dial 2 and the desired number. However, if you need to call home from your office, e.g. to tell your wife that you'll be working late, it would be a problem. Of course, you could avoid that by calling from antoher number (if your office has more than one line) or your cell phone, by calling to another number (cell, landline or SIP number at home) or blocking the caller ID.