This will trunk two asterisk servers together. Once you have them trunked you can add dial rules to dial extensions on each PBX.

Here's an example of a configuration between pbx1 <-> pbx2

Goto the "TRUNKS" GUI screen.

PBX1
"Outgoing settings"

Trunk name= pbx2

Peer Details:
host=hostname or ip of pbx2
secret=12345abdce
type=friend
username=pbx2

Incoming settings
User Context = pbx1

context=from-internal
disallow=all
host=hostname or ip of pbx2
secret=12345abdce
type=user
username=pbx1

On PBX2

Trunkname= pbx1

peerdetails:

allow=gsm,ulaw
host=hostname or ip of pbx1
secret=12345abcde
type=friend
username=pbx1

Incoming Settings
User Context = pbx2
allow=gsm,ulaw
context=from-internal
host=hostname or ip of pbx1
secret=12345abcde
type=user
username=pbx2

Now add 8| in dial rules to dail 8 to access other server

Now, goto "Outbound Routes" and on pbx1, you add a route that will let you dial pbx2 and a route one pbx2 to dial pbx1.

So, for example, if pbx1 is extensions 500-550 and pbx2 is 600-650, then your pbx1 outbound route "Dialpattern" would be

6XX

then in the drop down box, you would select the "pbx2" next to Trunk Sequence.

In pbx2, you would goto outbound routes, in the dial pattern box you would put in

5XX

then the Trunk Sequence would be "pbx1"

So, now on pbx1, you dial 600, the 6. would match and send the call to pbx2 and ring extension 600.

Same with PBX2 when you dial 500, it would pass the call to pbx.