turning off of 3G network effect on US manufactured iPhones in Australia?

I have heard a report that US originated iPhone 11, X, XR and XS and below will not work on Telstra and Optus networks once the 3G coverage is turned off. This is because even though these are 4G LTE phones, voice calls in these iPhones actually use the 3G network. To be clear, the calls appear to use the 4G network, but the band used on these devices for voice is actually 3G.

Is Apple or anyone else aware of this?