To make it work, you have 4 tasks:
- figure what network ports are used by your PBX or the protocol
- NAT these ports from sys-net to sys-firewall
- NAT these ports from sys-firewall to the destination qube
- Allow the traffic for these ports on the destination qube
It’s not really hard, I wrote a guide for this Qubes OS 4.2 nftables / nft firewall guide
It’s also explained in the official documentation Firewall | Qubes OS (I updated it recently for Qubes OS 4.2)