I am solving such a problem.
In the picture I drew how it works for me VVN (virtual voice net) (landline calls).
The Huawei router has (VPN) VLAN 11 and gateway 192.168.11.1
The Cisco MX84 firewall/router has (VPN) VLAN 11 and gateway 192.168.11.4
(pink arrows in foto) If the PBX ( central phone box) connects to the Huawei geteway, the call works but is not visible on the network.
If the PBX ( central phone box) connects to the Cisco MX geteway, calls do not work but the PBX is visible on the network.
Must the VVN be separated from the network for security reasons?
I need the PBX to be visible for remote surveillance.
Please, Advise how it should be connected by default.