asterisk users - Oct 2004 - Call waiting question for those who know the source

I've tried to ask this before but didn't want to waste a lot of
bandwidth. However, I have to give the params in order to make the
exact question understood.

We have 

2 ZAP FXO connected to two phone lines(2xX100P)
3 phones connected to a TDM400P with FXS modules
Usually a caller will be on ZAP/2 and the first outbound line in the
g1 group is on ZAP/1.

There are two (main) call waiting scenarios:

1) ZAP/3 calls out on ZAP/1 and a call comes in on ZAP/2
2) ZAP/3 gets a call on  ZAP/2 and a call waiting signal comes in from
POTS on ZAP/2

It seems in both cases, the person hears the call waiting beep and can
flash to the incoming call.

The Dial string for all incoming calls on ZAP/2 during open hours is
"ZAP/3&ZAP/4", both phones ring properly when both are on hook.

It also appears that in both cases 1) and 2) above, when ZAP/3 is off
hook, ZAP/4 will not ring when a call waiting condition happens.

Some of you here must have had to go through the zaptel stuff pretty
extensively, can you explain or point me to an explanation that would
provide answers to the questions,

A) What is supposed to happen when asterisk handles a call on ZAP/1
when ZAP/2 is bridged with ZAP/3 ? Given the dial string (ZAP/3&ZAP/4)
ZAP/3 gets a call waiting beep and shouldn't ZAP/4 be ringing as well?

B)  What is supposed to happen when asterisk handles a call on ZAP/2
when ZAP/2 is bridged with ZAP/3 ? ZAP/3 gets a call waiting beep,
again, shouldn't ZAP/4 be ringing as well?

Both X100P FXO channels are 
pickupgroup=1

Only one line (on ZAP/2) has callwaiting from the telco

Both TDM400 FXS channels are 
callgroup=1
callwaiting=yes
callwaitingcallerid=yes
threewaycalling=yes
transfer=yes

Any ideas and pointers would be of interest!