I agree with @yusssufff on the “countries” question. Just considering EU countries (where Jolla sells the licences), the table listed at “ VoLTE provider support ” shows VoLTE not working for any of the tested operators in Italy, Latvia, Portugal, Slovenia, Slovakia, Greece; and nothing ever tested for Hungary, Luxembourg, Croatia, Cyprus, Malta.
The question whether this is a “country” or a “operator” problem is pedantic. I’m ready to believe these countries have VoLTE support for Android and/or iPhone, but how would I know and why should I care, I own a SFOS device not an iPhone. Functionally for me the situation is the same as “no VoLTE in country X”.
Personally this is my main point of friction with SFOS/Jolla. I trust Jolla is working on improving their support; but they’re not reporting progress so I cannot tell if they are doing something or they just ignore some smaller countries. (But they’re selling us licences so why would they ignore us?) I expected to be the duty of Jolla to provide users with the status of support with different operators and countries, so users can make their choices. Not relying on two simple users like me and @amaretzek in a 10 million people country to fill the wiki with “it does not work in Portugal". I’m not going to buy 10 SIM cards from all the different operators just to find out VoLTE still does not work with any of them (though I volunteer to request and mail some SIM cards to Jolla if that helps making VoLTE work better). In any case, Jolla certainly knows more than I do and it wouldn’t cost them to give a specific answer for users residing in countries where the VoLTE support on SFOS is known to be problematic.
And to conclude back on the topic, there could be a warning at download time, saying VoLTE has been enabled by default in this ISO but you are advised to untick it for now if you live in Italy, Latvia etc.