The author of the article claims AMTA is not a reliable indicator of VoLTE compatibility:
Additionally Tools like the ‘AMTA 3G Lookup Service’ are not accurate or reliable. AMTA knows this, hence why you need to agree to a long list of terms & conditions before using it.
This new ACMA policy is deeply disturbing because many consumers, including myself, own fully functional devices that can make VoLTE calls and emergency calls, yet are deemed “incompatible” by the telcos.
These are devices with the exact same hardware and software as ‘officially supported’ devices, the only difference is the telcos didn’t sell them.
In fairness it is a poorly written and confusing article (the guy is not a writer by profession, just a telecommunications expert trying to draw attention to the 3G shutdown). I shared the video just in case people had a hard time understanding the main takeaways, since Hugh Jeffreys did a good job summarising them.
Even after running the extra test on Hugh's app it's still not clear if my phone will work. I have an unlocked Oppo purchased at JB HiFi and I'm using an ALDI sim so I'm on Telstra's network. It's not clear if it'll work even though I'm using VoLTE because of the above combo.
I just gave up and bought a new (secondhand) phone at the beginning of this year. I returned one of my older phones (Galaxy Note 9) to stock as a backup but I'm not even sure if that will work now because it's a grey import (even though it has working VoLTE currently and supports all Australian network bands). It's a bit sad that I have this little collection of otherwise great older phones that still receive updates to their custom ROMs but will now be useless as phones.