Even the SoC is not listed in the specs yet
But since the hardware is Jolla’s, they can enable VoLTE support, as they did with Jolla C2 a year ago (see [Release notes] Tampella 5.0.0.43 for Jolla C2)
Yes, it depends on the carrier
Those in Germany and France are usually stricter, while networks are generally fine in Jolla’s homeland
I don’t think Jolla could realistically negotiate with every individual carrier