The idea of a queue, as opposed to, say, a ring group, is that the caller is expected to (eventually) reach, an agent. The initial recording that they reach, when all agents are busy, tells them just that, but, allows them to wait while for one to become free. Reaching a busy signal just means they have to call back, which can be annoying, especially if it happens a number of times. If callers don't want to wait in the queue, they are still free to hang up, and try again later.
So, I suppose, if you really want them to reach a busy signal, you could have a busy signal as your initial queue recording.