asterisk users - Nov 2005 - "CPE does not support Call Waiting Caller*ID"?

I'm having a strange problem that I can't quite explain.   I feel  
like I'm missing something simple but I can't quite find it.

I'm not getting call-waiting CID whenever the incoming call is  
delivered over IAX.  However, when the same caller, coming in over  
IAX, hits an empty ZAP channel, the channel rings and the CID *is*  
delivered as expected.

I hear the CID bleep-boop, but the display indicates nothing and  
asterisk spits out "CPE does not support Call Waiting Caller*ID."

If I reverse this and have the initial call come in via IAX, ringing  
the ZAP channel, and the second call coming in via ZAP or SIP, the  
initial call shows the correct CID, I answer the phone, then hear the  
CID bleep-boop, the display indicates the correct CID, and asterisk  
spits out "CPE supports Call Waiting Caller*ID.  Sending Desk Phone/ 
0202".

I can't for the life of me figure out what the difference is.  It  
seems that if it can do one it can do the other, so I must have some  
configuration missing somewhere.

Inside "zapata.conf" I have:

usecallingpres=yes
usecallerid=yes
cidstart=ring
cidsignalling=bell
callwaiting=yes
callwaitingcallerid=yes
threewaycalling=yes
transfer=yes
cancallforward=no
callreturn=no

This is on Asterisk-1.2 out of CVS, fully up-to-date.

I have two ZAP channels, 1 coming in from the telco and 2 going out  
to an analog phone.  I also have a SIP desk phone (which always shows  
the right CID).

Anyone have any ideas?