asterisk users - Apr 2005 - Asterisk acting as PBX + SIP Proxy ... possible?

Hello,

I'm in the process of implementing the following setup

External SIP phones at another location(s) (nat = yes)
       |
       |  Analog phone line
       |  |
|------------------
|ext if 142.x.x.41   
|
|Asterisk   
|
|int if 192.168.0.1
|------------------
      |
Internal SIP Phones (nat=no)


Excuse my ASCII art ... if you cant see the diagram I'm basically doing the
following: 

- There are some phones on the LAN, and some other phones on the internet
side
- Both sets of phones use Asterisk to make calls between each other as if
they were all on LAN and to the phone line.


Is something like this going to work reliably?  Or will I need a second
central server to act as a proxy.  The reason I'm asking this is that I have
been able to make this setup work but am having some strange registration
issues whenever my external sip phones sit behind Linksys router (I get 403
forbidden) ... when I use some other router the stuff seems to work.  But
I'm worried about reliability since I read recently that "Asterisk is
not a
proxy" and I'm definitely using it as an outgoing proxy in this case.  

http://www.voip-info.org/wiki-Asterisk+SIP+not-proxy 

Has anyone successfully created this kind of setup before? (having Asterisk
pass calls on both LAN and WAN side?)
Do you have any hints for me to get this 403 forbidden error figured out?  I
think it might have something to do with FQDN - but the strange thing is
that it happens only behind Linksys
And if I do need an outgoing proxy which proxy do you recommend?

Thank you,
Tomas