DUNDi Configuration (no valid response)

Freepbx / Asterisk 1.8, installed from tarball.

I’m using DUNDi to connect two different servers, and I can call each server’s extensions internally. My “extensions_custom.conf” file looks like this in both servers:

include => ext-local
include => ext-intercom-users
include => ext-meetme

Server1 is hosting extensions 1XXX
Server2 is hosting extensions 2XXX

What I want is that, say for example, someone calls Server1, they will be able to dial extensions on Server2, and viceversa.

The mappings in DUNDi.conf on both servers look like so:

priv => dundi-extens,0,IAX2,priv:${SECRET}@xxx.xxx.xxx.xxx/${NUMBER},nopartial

The problem is that when I dial an incoming call from an external number, and at the IVR start dialing the extensions on the different server, I get the “we have not received a valid response” error. I’m not really sure what or where I should add/change things for this to work, so if anyone has any insight on this, it would be much appreciated.