When on the phone with your present setup, a caller should get your GV voicemail. If not, please report what happens.
GV does not support "early media", so the only way for a caller to hear a busy signal would be a fake one. For example, you could set up CallForwardOnBusyNumber to point to a free VoxOx or Callcentric number, and upload a voicemail greeting there that sounds like a busy signal.
Of course, you could use a service other than GV that supports giving a busy signal.
But I must ask, why would you possibly want this??? The message to your callers would be "Instead of allowing you to leave a message, I am forcing you to repeatedly retry your call, until my wife gets off her two-hour call."