Mark's suggestion is good, and I see your customer's concern, but I'd like to suggest an alternate.
Presently, someone calls into the queue and hears hold music, then when an agent is available the agent rings and answers.
With Mark's suggestion, an audio file of ringback tone is played, which would then be followed by hold music and then the next available agent.
Either of these is likely to confuse callers, as neither identifies the company or gives an indication of the call flow or what to expect next.
In my opinion, rather than an audio file of ringback, suggest that your customer record a greeting identifying the company and instructing the caller to wait for the next agent. Load that file per Mark's instructions above.
In this scenario the caller will hear a voice identifying the company and instructing them to wait for an agent, then hold music if no one is available, then the agent. This is the typical and expected behavior when calling into an ACD queue for most callers.
