Mmmh, typically nothing special, just connect a computer via one on the LAN ports and you should be fine (I am not sure whether it is possible to do the first login vie WiFI, at least OpenWrt stock defaults to no password, but disables the WiFi until after the first login via a LAN cable the user was forced/asked to create a password and to activate the WiFi, not sure about turris OS, as I installed that years ago and it seems to simply carry my configuration along flawlessly (good job, team turris, thanks!)).
I am not 100% sure, but what I would try to do is:
- log into the “LuCI OpenWrt advance web configuration”
- navigate to Network -> Interfaces
- click the “Add new interface…” button
- Give it a name, e.g. VoIP-WAN
- under Protocol select “DHCP Client”
- under Interface, add eth2.20 on the last line saying “custom”
- click “Create interface”
Now that should allow your router to get access to the VoIP VLAN of your ISP, but you still need to make this accessible on your internal network.
I believe the easiest solution would be to bridge VoIP-WAN with one of the LAN ports of the switch and use that as dedicated VoIP network port on which to access your VoIP base station. But I have never done this with the omnia’s DSA architecture so have no first hand experience and will refrain from proposing exact methods (without having tested them they will almost be guaranteed to be wrong).
Alternatively would be to try to distribute VLAN 20 over all ports, but then each device intended for VoIP needs to be configured to use VLAN 20, and I am not sure whether the omnia’s switch will allow untagged and tagged packets on all ports.
Alternatively, contact your ISP and ask why they can’t simply follow the incumbent’s model and allow access to their SIP servers over the PPPoE negotiated link…