You’ll find that with any major VPN. The IP addresses they use to proxy your traffic eventually get flagged and blocked by lots of major players. Which is why VPN companies cycle through them quite often. As others have said, you’ll either need to switch servers (and thus ips) or figure out another path.
I don’t use mull but most have a way to exclude a given url or site from the tunnel if you need it. i.e. the site will work for you but it’s coming from your own IP and unencrypted.
I’ve used wi-fi calling fairly extensively mostly because I’ve lived in areas where there was zero cell service but ready access to internet (via Starlink or other wireless forms of it). One thing I do know is that my phone co. requests that I fill out a form specifying where I am living currently (whilst using it) so that if I ever need to contact emergency services they’ll have a better idea of where to route the call to. For instance my phone number originates from Western BC but I could potentially be using wi-fi calling from anywhere in the province. I mention this to say, it appears my telco doesn’t have a way to triangulate me with this service.
I can further attest that wi-fi call & text reception still works fine when I have a VPN running on the router that my mobile device is connecting to. Make of that information what you wish.
Though that I have read that wi-fi calling is atrocious for privacy reasons that I have not followed up on. Given the above I’m not sure how or why that would be the case, but basically if I’m in an area with cell coverage I turn it off. I’ve always meant to look deeper into how or why it might be bad (or worse) in some way.