asterisk users - Jan 2005 - One way call when the * server and phone in a local network

Hi everyone,

I started playing with Asterisk server a few days ago. So far, I only have 
made it 50% work.
Here is my situation, IP phone A and Asterisk server are in a same local 
network behind an ADSL router (public ip = a.b.c.d), and the * server is set 
as the DMZ host of the router.
IP phone B is in another network.

  IP phone A
  (10916)         ---> ADSL router<-->internet<-->
Rotuer2<--IP phone B
  asterisk server         a.b.c.d                               (10920)
  (192.168.1.2, DMZ host)

I can place call from phone A(10916) to B(10920) without any trouble.
But I cannot make a phone call from B to A.
I tried  "sip debug peer 10920" to check the SIP message flow, and
found a
problem.
Here is part of the log:

.....
From: "10920" <sip:10920@a.b.c.d>;tag=zrgrUBMCJ7bVPtqC
To: "10916" <sip:192.168.1.2@a.b.c.d>
....
SIP/2.0 404 Not Found
....

It figures out a wrong target sip address "10916"
<sip:192.168.1.2@a.b.c.d>"
for some reason. "sip:10916@a.b.c.d" would be better?

When place call from 10916 to 10920, it looks fine:
......
From: "10916" <sip:10916@192.168.1.2>;tag=vmu4DFbjvCRUAuD8
To: "10920" <sip:10920@192.168.1.2>
......

I reckon because the * server is the DMZ host, which may affect the traffic 
to phone A that sits
in the same network. Is that the case? Any advice would be grealty 
appreciated! Thanks.

Cheers,
Dan

p.s my sip.conf

[general]
context=default		; Default context for incoming calls
port=5060			; UDP Port to bind to (SIP standard port is 5060)
bindaddr=192.168.1.2		; IP address to bind to (0.0.0.0 binds to all)
srvlookup=yes			; Enable DNS SRV lookups on outbound calls
tos=0x18
disallow=all
allow=g729
allow=gsm
allow=ulaw
allow=alaw
allow=all
;If one is willing to forward some ports to the machine running asterisk, 
adding the following
;lines to the [general] section of sip.conf has been known to work;
nat=yes
qualify=yes

externip=a.b.c.d
localnet=192.168.1.0/255.255.255.0
;canreinvite=no


;the IP phone at home
[10916]
type=friend
secret=lucy
host=dynamic
callerid=Lucy <10916>
disallow=all
allow=g729
allow=ulaw
allow=alaw
nat=yes
qualify=yes
canreinvite=no

;the IP phone in friend's home
[10920]
type=friend
secret=dan
host=dynamic
callerid=Dan2 <10920>
disallow=all
allow=g729
allow=ulaw
allow=alaw
;allow=gsm
nat=yes
qualify=yes
canreinvite=no

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