asterisk users - Mar 2008 - Central Asterisk with remote 'trunking' asterisk gateways

Dear all,
Wanted some help on a solution we wish to deploy. There is a central  
Asterisk server which is connected by some 30+ remote sites. Each site  
has a substantial number of users within them (200-300). What I wish  
to do is save on bandwidth by trunking connections between those sites  
and the main site. I can use a flash based solid state device for this  
purpose (Xorcom or ASterisk appliance).

However, I do NOT wish to create user accounts on the remote devices  
(for central management). I want the phones to authenticate/register  
to the central server but use the appliance as a 'proxy' so that all  
calls are sent through it only (and IAX2 trunked, obviously). Is this  
somehow possible or do I HAVE to register my phones to the local  
appliance?

-- 
Faraz R Khan
Chief Architect
Emergen Consulting Pvt Ltd
www.emergen.biz