Domain name filter
The simple filter on our PBX for the domain name was that we were throwing the baby out with the bathwater. It was simply not possible to define exceptions. That made it practically impossible to use the feature in a real deployment. If the system administrator white-listed IP addresses, traffic from there would still be ignored if the domain name did not match. Adding that back in to the filter solves many of the trunk related problems, as admins can just add the IP addresses for the trunks.