Problem with remote extension

Hello all !

I have a tricky situation with a remote extension - let me tell you the setup :

  1. I have two networks, each one has an ADSL-router
  2. in each network there is an asterisk box (elastix 1.1)
  3. in network 1 I added a sip extension (6000) and I configured the NAT and the asterisk server and this very extension conforming to this article :
  4. in network 2 I use x-lite and I registered the extension 6000. I also put the machine that holds the x-lite application in the DMZ.

Now, when I do a call from the 6000 extension in network 2, everything works fine and the user may call as if he would be in network 1. BUT : if I call the extension 6000 from another extension that is registered in the asteriskbox 1 (in the network 1), then most times I get a message of the kind “service is not available” (depending on the softphone application).

I have the impression that there is some port conflict with the second asterisk box (in network 2), which is not involved in this scenario, but I just mentioned it in order to give you all the details. But I am not sure that it is a port conflict. So if it is, is there anybody who has an idea how to get it fixed ? And if there is another reason for this problem, what could it be?
Edit : I just did the test to stop the asterisk server 2 in the network 2, and now everything works fine. So my question is : how could x-lite and asterisk co-exist in network 2 without the described problem.

Thanks to all who could give me some hints on this tricky problem.