@Mynorx As you said, things were working fine, until you changed trunk server ip.
In general, if you have freepbx being registered, this also maintains states and there isn't much to configure. (unless you also need qos, but that's something to take care at a later stage.)
No there is no security issue involved with outbound nat.
Its a good idea to disable freepbx firewall unless you also have internal threats consider.
(freepbx fw is designed to protect the product living on the wild internet.)
At least stop it while you are investigating. Makes life easier.
Since you are having issues with rtp, try portforwading the rtp range configured on freepbx from the internet facing interfacing to your host internal ip
Then go to diagnostics states and type the ip of freepbx and see if you have relevant states. Also use the trunk ip and see what happens.
pfsense works nicely with freepbx in many production environments.
ps do post your rules, there shouldn't be any discrepancies with the ip's you mention.
You are probably doing something wrong...