One box as gateway, 2 as clients

[atux_null]

hi. i have 3 boxes with the following names and extensions:
-Gateway with all the sip trunks to the outside world and extension numbers of 3XX. 5 DIDs(eg 123456780-4) will be assigned to 308. 5 DIDs(eg 123456785-9) will be assigned to 309.
-Box A where it has extenions numbers 4XX
-Box B where it has extenions numbers 5XX

i would like boxA to register as extension 308 in Gateway and BoxB as extension 309 in Gateway.
BoxA and BoxB will able to make/receive calls through Gateway only
I am a bit lost on the configuration

[cynjut (Dave Burgess)]

This is the hard way to do this. You should get an Session Border Controller. Look for Kamillo (sic) for a box designed to do your external part of the connection. Each of the back-end boxes then register with the gateway as trunks. Much simpler to maintain and configure.

[atux_null]

it is really hard and very costy to invest in an extra machine (Kamailio). we need to make it work as is with the current machines.

[arielgrin]

Then use trunks to link all the FreePBXs, either SIP or IAX, don’t register them as extensions. That is what trunking is supposed to be used for, IMHO.