Based on the comments by @dos, I’m guessing that Purism is waiting on BroadMobi to add VoLTE support, which he indicates is a matter to configuration and international testing. If that is the case, then a firmware update can fix it, but of course only BroadMobi can provide that.
I would wait until Purism is ready to ship Evergreen and see what Purism says before making any decisions.
Another issue is whether you have a cellular provider that has a whitelist of allowed phones on its network. You might want to check the policies of your provider. In the US, this is a big problem. It isn’t where I live in South America.