It's the fault of the carrier
I'm with the Telstra network (via a third party reseller)
A year ago, they sent me regular SMS saying that my phone (Samsung A5) does not support VoLTE and will soon cease to work after 3G was shut down.
As I got closer to the deadline - all my outbound calls were prefixed with a voice message stating the same as above.
Why is it that Telstra were able to detect my phone's incompatibility and advise me well in advance of the 3G shutdown and yet Optus could not ?
The other mobile phone carrier is Optus who are are literally staffed wall to wall with MORONS.
TPG is not a mobile phone carrier and relies on partnerships with other telecommunications companies like Optus and Telstra.
So their customers who are on Telstra were probably notified - but obviously not for flOptus.
As for Samsung, they could have issued a patch for my 2017 A5 to support VoLTE but they didn't because they wanted more sales.