If callers first go to a queue, then while on hold, they should hear music (after they hear a message explaining why they are holding. That is how queues are supposed to work. If you give callers ringback tone, many will hang up after a time, I know I would, thinking that somewhere a phone is ringing but no one is there to pick it up. People (customers) should not have to wait (endure) more than 20 to 30 seconds of ringback, after that you risk loosing them.