I did a bit of playing with this this afternoon.

The entry in my extensions.conf is now :

exten => 29043,1,Playback(/etc/W3HZU/announce/SPECIAL/YorkStar_Radio_Network_v2,noanswer)
exten => 29043,n,rpt,29043
The "noanswer" option plays the file without answering the call.

If you don't add that, it seems to keep repeating the file, and holds the node (and all others connected) in transmit.

Details here : http://www.voip-info.org/wiki/view/Asterisk+cmd+Playback