I emailed RingCentral support and asked this question:
On "Call Queues" > "Call Handling & Members" > "Wait Settings" > "When maximum wait time is reached, send caller to", my only option is Voicemail. Is there a way I could change that to Forward to an external phone number instead?
Their reply was that the feature I want is currently not available. Does anyone know of another way to accomplish what I'm trying to do?