SIP and IAX trunks working one way only

I have an old FreePBX v2.11.0.42 that I am trying to trunk to a newer FreePBx v12.0.22. Regardless of protocol (SIP or IAX2) calls only work from the new to the old PBX but not viceversa. I assume must be something related to security (call is declined code 603) but I disabled iptables and fail2ban and problem is not resolved.
Any ideas I may look into?

Possibly not FreePBX, but the older version of Asterisk you have on the older box.

Of course this only applies to IAX2 calls.

I have a working IAX2 connection between (Old) and 12.0.62 (New) working fine.

Tail both servers /var/log/asterisk/full log file and check if you have a successful connection between both servers. I used this example setup

Thank you all for your suggestions.
It was not a trunking problem as I posted.
I upgraded all modules with security warnings on the welcome page, but I missed to upgrade Call Recording module. That caused an error with every call. See this thread:

Extensions not working after update.

Problem solved. Sorry to mislead you with my wording of the problem.