Yes, I think this is an old fashioned concept. I'm interested to know what you plan to gain by presenting a busy signal as well. I can suggest a couple of options.
Option 1:
If you don't want anyone to leave a voice mail then replace your Google voice mail message with a busy signal. This will trick them into thinking they reached a busy phone. If they stay on the line long enough they could still leave a message but most people will hang up immediately when they hear the busy signal.
Option 2:
If it must be just when you are on the phone maybe you could replace the main OBi auto attendant recording with a busy signal. Then set Call Forward On Busy to forward calls to Auto Attendant. That will give them a busy signal and no ability to leave a message.