I agree with lretrievers.
Also, consider whether you have enough ports to dedicate to this function. This type of announcement was possible with Symposium 4.2-CallPilot 2.0, and from the beginning with Symposium & Meridian Mail. But it does require dedicated ports (they cannot be used for regular voicemail, and they will be in a separate queue).
If you do have ports to spare, though, you might consider using either the position in queue or a general statement (your expected wait time is less than x, or we are currently experiencing extended wait times/high call volume).