Unable to route the inbound calls

I am a newbie regarding Asterix.

We have three servers of TrxBox Free PBX installed.
Version : Asterisk 1.2.13 svn rev 47264

Current scenario

Main Server
BRI card installed, incoming and local NWD calls are routed through it.
Most of the extensions are configured on it
3 dgits extensions

Second Server
4 digits extensions
For NWD calls use route for main

Third Server
Located in separate building
Few extension are configured there.
4 digits extension
For NWD calls use route for main

4 ZAP Trunks for BRI channels

UAN and 4 DID’s are configured on BRI card one interface.
All servers are interlinked with each other through IAX2 Trunk and can easily make calls among each other.
Users on other servers make NWD calls through Main Server

Extensions configured on server 2 and 3 can’t receive the calls from NWD interface.
We have the option enabled that the caller on DID through IVR can dial the extensions configured on Main Server
Additionally, need to change the option, if caller on DIDs want to dial the extension it should be dial # first
Make the IVR options that calls forward to another server extensions or ringgroup.

I am very much thankful to you for your cooperation and assistance in this scenario.