asterisk users - Feb 2006 - Queus seem to work different between IAX and ZAP channels

Queus seem to work different between IAX and ZAP channels. I have a IAX 
trunk today which I want to switch over to ZAP lines. I have a menu 
option for a caller to be able to "locate" as person. What happens is
a
queue is setup with a ringroup in it. The ringroup is an external # with 
a 15 second timeout. With the IAX trunk this works. A caller hits the 
option, goes to the queue and listens to music on hold while the extenal 
# rings for 15 seconds. If no one picks up it puts the caller in the 
persons voicemail. With a ZAP trunk the queue calls the external # but 
does an xfer. So the caller doesnt hear music on hold but the extenal # 
ringing. The timeout doesnt apply anymore either so it seems it is doing 
a xfer. Any ideas?

-David