Remote server madness - SIP trunk is working when disabled and not when enabled

Hello everyone,

I am trying to connect my remote server to the main one. I have tried setting up IAX trunks to connect the two (there is plenty info online on how to do that), but given up after I was getting “it’s all busy now” messages no matter what I did.

The only functionality I need is that when a user dials 9500 on the main server, he gets connected to the conference room 200 on my remote server (remote server got public dynamic IP, with DDNS set up. Main server got fixed public IP).

I set it up in the following way: created Chan_SIP extension 9500 on the main server. On the remote server, I created a SP trunk that registers on that extension (see pic enclosed.) Inbound routing on the remote server leads all calls with DID 9500 into the room 200.
So far so good.

Here is what drives me nuts: the forwarding does not work when the trunk is enabled, but as soon when I disable it and apply settings, it works perfectly, routing 9500 calls on the main server into 200 on the remote FreePBX.
If i restart the remote server, it stops working, then I need to go: “untick DisableTrunk, save, apply config, tick Disable Trunk, save, apply config” and it works again.

Any ideas on how to make it work consistently?
I use Stable 6.12.65 with Asterisk 13 on remote server, FreePBX 12 with Asterisk 11 on the main one.