I have a SIM-free Galaxy S9+ (SM-G965F/DS) model. And in my home country (India) VoLTE works fine on it. I'm moving to Japan soon and was thinking of getting a Japanese SIM (The name of the carrier is au) . But I have read many posts about peoples phone not working with au VoLTE service (voice+sms service not working).
Is it possible that different countries use different VoLTE standards and my phone might not support au VoLTE?