This has been handled differently in the UK from a regulatory point of view from an earlier time in the rollout of 4G/5G networks, so things seem to have worked out better.

In the UK, carriers are only allowed to provide coverage in any given area to a phone if it is able to make an emergency call (999/112) in that area.

i.e. if the phone says you have a signal, you must be able to make an emergency call.

So, for a phone to actually show you as being connected to the network:

a) There must be 2G/3G coverage available for a CSFB call to take place.

or

b) The phone must support VoLTE (including for emergency calls).

Newer LTE (and 5G) spectrum deployments, like Band 20 LTE (800MHz), are being used to provide coverage in rural areas - even in places where there is no 2G/3G coverage, therefore these newer bands are often only made available to devices that support VoLTE.

If you have an old phone, that doesn't support VoLTE (including for emergency calls), then it will only connect to 4G networks in areas where there is still an overlapping 2G/3G layer.

This approach means that the carriers have had an incentive to make all devices support VoLTE with emergency calling. It has also been possible for the carriers to promote the 800MHz coverage as only being available on newer phones with VoLTE.

Thus much of the problems that Australia is having have been avoided.

Also:

Data-only devices aren't included in this requirement, as they can't/don't make emergency calls.

Roaming devices aren't affected (as far as I know).

UK carriers support VoLTE roaming in the USA, given the lack of 2G/3G networks for CSFB.