dialin server configuration!!

hi all
i am setting up an intranet mail servers in my office using dialup.Now i had
setup the headoffice server and also servers at branches. Now all my branches 
are able to connect to my server at head office and relay mails. The problem 
is that when i am trying to connect to my branch from head office , it gets 
connected but not able to ping each other.Ie. the server do not compromise
the gateway.  is it possible to give a fixed ip based on usernames ? in short, 
if any body can give me an idea on how to configure two dialin servers which 
can dial in each other and communicate each other.

regds
Narayanan Subramaniam
System Admin