Have you any experience/suggestions with replacing the Vodafone supplied router with a Draytek router and still being able to use the bundled VoIP service?

The standard setup is an ONT on the wall and then a Vodafone Gigabox (actually a THG3000G with custom firmware).  The Gigabox has an ATA built in and two phone sockets on the back.  They don't expose many details re:connection or VoIP on the config screens.

I have replaced the Gigabox with a Vigor2927ax - if you are doing this, there are a couple of gotchas-
- You need to ask their vodafone chat service for the user name and password - they have no clue beyond this for any other settings
- Setup the WAN as PPPoE with supplied user ID and password
- my fibre is from CityFibre, so I have to enter a VLAN tag of 911 (service site)
- change the MTU to 1492

I'm not 100% I have all the settings correct yet as the wired connection via the Gigabox is faster than the Draytek.  The Wifi is also faster on the Gigabox, despite the Draytek being a higher standard (I'm working on these)

Now the problem - I can only use if the VoIP service if I use the Vodafone supplied Gigabox.

I have tried it plugged into a LAN port (to see if it would just use it DHCP) - no luck.

I have tried to setup a port on the Draytek either as passthrough PPPoE or using VLANs with the DrayTek LAN port at one side and the Gigabox WAN port at the other - I've not been able to get this to work.

Has anyone been able to get the Vodafone VoIP service to work using the GIgabox (they won't give the setting, so I can't use an ATA.  They tell me the reason is "security" - security by obscurity?)  or by any other means?

Thanks!

Notes:
Helpful setup note - https://www.draytek.co.uk/support/guides/kb-fttp-wan-setup
Someone from NZ who got it to work, but this isn't working for me - is it the VLAN ID? https://support.snappernet.co.nz/support/solutions/articles/5000781565-using-a-vodafone-router-with-built-in-ata-behind-a-draytek

I’ve no experience of trying this but I don’t think it will be easy.

Don’t assume overseas Vodafone VoIP services are similar to those in the UK.

Vodafone are not obstructive with you using your own router for Internet connectivity but I believe that there are legal responsibilities about knowing the address a landline is used at relating to 999 calls which discourage making it easy to use your own hardware and their VoIP service.

The BT Digital Voice service is similarly difficult to use behind another router but I’ve seen in passing people writing up how they did it. A similar approach may be possible with Vodafone’s service but I think you will find it challenging.

I’m also not that impressed with the WiFi on the 2927.