Spy QUEUE - Help Please

am using Agents and Queues concept of asterisk for a call center to service the customers with agents.

Now I have a different requirement that, say for e.g Agent1 is on call with Customer1, the other customers in queue (Customer2,Customer3, customer4) should hear (spy or conference listen only) what Agent1 and Customer1 converse. When Customer1 hangsup, then customer2 should be able to converse with Agent1 and the others 3 and 4 should be able to hear (spy or conference listen only) and so on.

There is only One agent in the queue.

May be to achieve this simpler, is it possible that we can put this Agent’s channel as hold music?

Any guidance is greatly appreciated.