When no phone agent is available to answer a call you can play your greeting to the caller and automatically move them to a queue where any agent can connect to the call when they free up.
Video Guide to Setup
Addendum to the Video
The addition of the Auto-Greet feature has slightly changed the layout of the configuration screen for Call Queueing. The Auto-Greet setting now appears at the top of the screen and the remaining Call Queueing portion is largely unchanged