SIP Trunk Call transfer to another PBX

Anybody that can help me with the following issue would be great:

I have two systems in my network. One is FreePBX and the other is a third party system PBX.
I successfully have the two talking to each other but I ran into an issue and hopefully someone can point me in the right direction.
In the other PBX, I have a TRUNK set up with an extension registered to FreePBX.
On my FreePBX I have created a trunk and a route to call the other PBX. Here is the issue.
When I dial into an IVR on the other system from a FreePBX extension, this IVR is set up to ask the user to dial an extension which is local to that PBX. The problem is that when I enter the extension to dial, the system actually tries to dial that extension on FreePBX and not on the local system. So for example if I tell it to dial extension 600 it refers back to FreePBX to call that extension and it rings extension 600 on my FreePBX instead of sending the call to a local extension.
Here is what the logs show on FreePBX:

== Spawn extension (from-internal-xfer, 600, 1) exited non-zero on ‘DAHDI/8-1’ in macro ‘dialout-trunk’
== Spawn extension (from-internal-xfer, 600, 1) exited non-zero on ‘DAHDI/8-1’

What I want the other system to do is to dial extension 600 locally and not call that extension at FreePBX.

I am not sure where to make changes.

Sorry If this sounds like a newbee question.


What is the other system and do you have a SIP trunk setup between the two or some sort of analogue or digital connection? Also is it the other system that is actually playing the IVR or is it Asterisk on your FreePBX system?

Thanks for your reply.
The other system is called FRONTRANGE IPCM.
It is an integrated voice software PBX system installed in Windows 2012 Server.
And Yes, I have a SIP TRUNK on FreePBX as well as one on the other systems. The other system’s TRUNK settings also registers to an extension on FREEPBX.
The IVR on the IPCM PBX answers the call and It runs it through the IVR script which in turn requests for an extension to transfer. When I enter the extension, it transfers to an existing extension on FREEPBX instead of the existing extension on IPCM.
Hope this answers your question.