Need help settings up a Sophos XG router on a DOT network

Basically i currently have a DOT system (Digital Office Technology) with a router that handles all phones so i can't get rid of it, i would like to setup the Sophos XG behind this router and also use it to do my VPN, is this possible? What is the best config to have the Sophos in.

I see setting it up in a bridge, the VPN isn't supported would this still work?

I've tried settings up the Sophos as my gateway and the DOT router behind this but i randomly get phone issues no matter what changes i implement.

Any help would be great.