fated:
I have a 6 Pro on Vodafone and no VoLTE. I put the Spark sim out of my wife's phone in my 6 pro and VoLTE was provisoned and the 4g calling selection in settings was available.
So carrier specific for VoLTE, not country limited.
Well the original article was about Google adding carrier specific settings into the Pixel 6 (and 7 I guess) so that a user in one part of the US when roaming to another part onto a carrier who had removed all their 3G cells, the user could still make voice calls and for international users. So they must have reached out to a number carriers both domestically and internationally to get their settings for VoLTE and insert them into the update.
5G is not so important so that might be why it doesn't work for Spark while VoLTE does.
While it doesn't work on Voda is anybody's guess - negotiations broke down, never started, who knows