Replacing ring tone with a message wouldn't be allowed in Oz anyway. The ACMA would drop on you like a ton of bricks if you did/could. You have to be careful even sending ring tone to an incoming caller too, the caller has to be aware when charging starts to give them the option of hanging up. There was a big issue some years ago where certain telephone systems with call queuing would put you into the queue, tripping charging, but then playing Australian ring tone so the caller didn't know they had started paying for a call which hadn't actually been answered.

I would be steering well clear of this one if I were you!