[asterisk-users] how to DUNDi branch office with area code?

[d tbsky]

hi:
   i am new to asterisk and dundi. we have some branch office which
will use asterisk in the future.  they will form a full-mesh structure
so every site can contact each other directly. i want to try setup
dundi, then we don't need to modify every pbx when a new site add in
the cloud.
   thanks to the great dundi document "caveman can do it" and other
resource in the voip-info.org. i learn the basic setup of dundi. but
i want to a little advanced setup with area code. like this:

  site HQ: has extension 101,102,103, and site HQ has area code 99
  site A: has extension 101,102,103, and site A has area code 01
  site B: has extension 101,102,103 and site B has area code 02
  site C: has extension 101,102,103 and site C has area code 03

we want to use "4" as prefix to call to the internal cloud. so user at
site A can call "4-99-101"  to contact extension 101 at HQ.  site B
can  call " 4-03-102" to contact  extension 102 at site C.

now i m confused about this structure with DUNDi. i don't know the
best way to setup DUNDi for this structure. i think maybe i should do
below when user  call "4-99-101"  at  site A :

 1. site A ask for dundi request  "4-99-101"  to site HQ
 2. site HQ strip "4-99" and look up 101 at local context
 3. site HQ return the destination to site A
 4. site A use the destination to call extension 101  at  site HQ

i don't know if step 2&3  is possible in dundi.conf. the example in
the internet didn't tell how to do this.  or there are better/standard
ways to do this?

thanks a lot for any suggestion!!

Regards,
tbskyd