At least Vodafone Germany intentionally blocks (or used to block) foreign IPs for their gateway. I'll always trust them to needlessly ruin perfectly fine technologies.
Fortunately, as far as I remember at least iPhones route VoWiFi traffic over a VPN, if any is connected, so that's one way to still use it abroad.
This is actually less unreasonable than it seems at first.
The Vo WiFi spec only defines handover procedures for networks that implement VoLTE, but VoLTE roaming was historically almost nonexistent, although the situation is improving somewhat.
This means that if you go out of range of your WiFi router, your cell phone has no idea how to request a handover from the network, and the call drops.
As noted by other commenters, some carriers (I think all of the UK carriers) use IP-based geoblocking to ensure that Wi-Fi Calling only works when using an IP address registered in the UK.
I think most EU carriers use geo blocking for VoWiFi. A lot of non-European carriers advertise VoWiFi as a cheap alternative for roaming.
For EU carriers roaming used to be a big business (Europe is probably the region where most traveling abroad happens, many small countries close together without travel restrictions). The EU carriers don't seem to be able to let go of crazy roaming charges (5€ per minute and more are not uncommon outside of EU).
I have a dual-SIM Pixel 7, the eSIM "slot" has a data-only subscription, the other slot has a pay-as-you-go SIM that I can make phone calls with (I make so few phone calls to actual lines, that having credit that I can top-up every few months is much more cheaper than paying monthly for free minutes). The PAYG SIM offers WiFi calling, and the phone appears to even offer "WiFi"-calling over the data connection, for a much better audio quality.
