asterisk users - Mar 2005 - Two (or more) Asterisk servers, routing calls

Hi everyone,

since I finished some hardware issues, now the real * configuration started.
It is my first attempt to get asterisk working and I am a bit confused.

The structure I am going to configure is quite easy:

The asterisk server is connected to a traditional PBX via S0.
When a user dial asterisk internal number followed by one of specific 
phone number (i.e. remote branch offices, so user dial 120 12345 (which 
120 is asterisk local number and 12345 is the remote number to call), 
asterisk should understand that 12345 is another asterisk remote server 
and redirect the call to the remote server IP address. That remote 
asterisk server must accept the call and divert it to a another 
traditional PBX and then make the analog phone ring.

in summary: Phone -> Analog PBX -> Asterisk -> INTERNET -> Asterisk
->
PBX -> Phone *

Who can give me some hints and advices to get this done?
I already read alot, not enogh surely. But since I am too much confused, 
I need some clear and surely right help.
This because I am not sure which way take.

For example:
SIP or IAX?
Should I use 'register =>' in sip.conf for both server, one looking
for
the other?
In which config file I tell * to "forward" to the PBX?

Thanks in advance for your help and patience.

Giorgio