Pre-Recorded Audio Over RingOut Call

My app uses RingOut to place automated calls. In my use case, I never really expect the "caller" to pick up the phone. I would like the "callee" to be played some sort of pre-recorded audio message if the caller has not picked up their leg. Is this possible using RingCentral's API? I see that I can create "Greetings", but how can I append one of these to a RingOut request such that it plays on pickup?