OK, so the combined mode for the VDSL port was a false start, so I revisited the original "how to get VOIP working when setup in bridge mode" problem. As j0hn posted, we haven't been able to find a way to get the 8924 to see a DNS. Also, by default as well, there's no way to specify a different default route. So, first two steps are as per the time server access
Specify everything with IP addresses, not names
Setup static routes to specify the router as the route for the IP addresses.
So, I specify the VIP Service using the IP address for sip server, registration server etc.
Create a static route to the IP address of the sip server with the gateway as the router.
So, that's as per the time server approach. As I mentioned before, we're still stuck because the SIP service has a setting specifying which port to bind VOIP to, and it only lists WAN ports. So, it won't activate VOIP unless it see one of the WAN ports active - and it can't see the WAN port since it's not in the same interface group.
However, I saw something very interesting in the manual. The screenshot there for the list of ports to bind includes LAN as well and the WAN ports, but the UI now doesn't have LAN. So, looking at the command line, Voice Show tells you the list of commands .... Oh ...
voice set BoundIfName ServiceName port
is one of the commands.
So, let's just give it a try
voice set BoundIfName Voipfone LAN
Ah - no error message. Disable and re-enable the SIP account in the UI - and hey presto - VOIP now registers and works.
So, you need to setup a static route for the sip server via the router gateway, use IP address since there's no DNS, make sure the firewall is turned off, and get it to look for an active LAN port rather than WAN port. The only other thing I noticed is that it doesn't auto connect at that point. You need to go and mark the SIP account as inactive then active again to kick it into action, or maybe do voice stop, voice start at the cli (didn't test that).
Hope that makes sense....