anononetwo i mostly use Wi-Fi calling and use a specific wireguard container as a gateway on my proxmox host for my phone only. Mostly to prevent potentional vpn leaks on-device, and also to use Wi-Fi calling routed through a vpn. I use this now for almost 2 years i would say, and never had any issues. In some unique instances, there is some delay/latency issue, but almost never.
In my case, for Wi-Fi calling to be able to work through VPN, i found out i had to have exit node ip originating from my country, presumably a check on providor side. Any ip not originating from my country, and Wi-Fi calling won't work.
I'm not sure how Wi-Fi calling could 'bypass' the VPN on the next hop, if configured properly. So, Wi-Fi calling routed through VPN on router or next hop works fine, maybe your providor requires exit node ip from originating country aswell, but i'm not sure this is standard practice wordwide.