I'm not a GV expert, but I think it will never return a busy even when the phone is busy. That's the way it works and after about 25 seconds it will go to VM. However, you can route the call to another service that will produce a busy. The easiest way I know is to route the call to NoMoRobo with your CallerID. I don't know if you are familiar with NoMoRobo. It's a free service that blocks Robo calls. You normally would route calls to your phone port and NoMoRobo. If the CallerID is in their database it will answer the call with a message and the call will ring your phone once and then stop. If it's not in their database, a busy will be sent and your phone will keep ringing until answered.
You would route the call to NoMoRobo with your CallerID when the OBi202 phone is busy. Your CallerID won't be in NoMoRobo's database. The caller will hear 1 or 2 rings and then a busy. I did NOT test this. I do know calling NoMoRobo directly will produce a busy. Is this what you want? I need to know if you have a non-GV trunk defined and what SP it's defined on.