asterisk users - Oct 2005 - Have IAXy signal busy without losing ongoing call?

I'm using an IAXy witha current CVS-head build of Asterisk.

The IAXy has an extensions.conf entry somethng like this:
exten => 1,1,Ringing
exten => 1,2,Answer
exten => 1,3,Voicemail(u1)
exten => 1,4 Hangup

This works fine for calls routed to extension 1.  But if a second call is
routed to the IAXy while it's already busy, the first call disappears from
the IAXy and the second one goes to voicemail.

Asterisk says that the second call is accepted by the IAXy and then it
determines that the extension is busy.  But when the second call is
accepted, the first silently disappears.  Is doesn't seem to get hung-up
until I hang-up the phone attached to the IAXy.

Is there a way to have Asterisk dtermine that the IAXy is busy without
interrupting the ongoing IAXy call?

frank