asterisk users - Oct 2005 - Asterisk Nat solution for such scenario?

Hi, everyone. I have such a setup:
Asterisk--->FireWall-------<internet>---------->FireWall
|------>Cisco7960

|------>Xlite

And I followed the instructions I got from the internet, change the 
sip.conf in asterisk like this:
[general]
....
nat=yes
extexternip=203.92.75.87
..
[cisco]
.....
[xlite]
.......

Finally I manage to get Xlite works perfectly, while for Cisco7960 I 
have to disabled the sip password, then I can make outgoing call, but 
still cannot receive
incoming call.

Is there anyone encouter the same problem before? any ideas how to solve it?

Thanks,
Liangliang