PVE Firewalls and SIP

I am having a very strange problem with firewalling a VM, which is a virtual PBX. I've spent a bit of time troubleshooting this and I simply cannot figure out what is happening. I consider myself pretty well-versed in PVE and FW, literally hundreds of VMs across a few clusters, many with complex firewall setups that work just fine. I've made this issue as basic as I can. This is a Linux VM with a VirtIO network card.

If the "Firewall" checkbox is NOT ticked on the network card, everything...

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