I just read this column and like to suggest a simpler solution. I have a couple of similar setups with multiple LAN segments, sometimes with VLANs. They basically share the PBX and the usual groupware stuff like calenders.
I know that OPNsense does exist, but I have never used it. My stuff works with pfSense, where it is now pretty easy to install packages from other repositories. Of course, one needs to be careful when updating the system to keep all packages in sync when the upstream repositories change.
Having said that, why not install Asterisk in the router/firewall right away? No NAT issues, multiple LAN segments as easy as pie, perfect control of packages.
Maybe OPNsense does allow to do something similar.