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So they'll just fall back to 4G then, which always sends the IMSI in the clear on initial attach?

5G Standalone networks don’t have 4G to fall back to. 5G Non-standalone networks are essentially 4G networks with a 5G RAN, so SUCI remains optional and most core vendors don’t support it.

That's not what 5G standalone means, as far as I understand.

The network I'm using supports 5G SA in some cells, but my phone definitely still falls back to both 4G and 5G non-SA in some areas where it's not yet available.

And even if 5G SA were available everywhere, there's the concern of roaming.

Correct, your phone needs to actually re-connect between the two networks. It's a whole new session and you can't handover between 5G SA and 5G NSA/LTE networks. There are some configurations that make this not much of an issue, but technically they are totally different networks.

You can definitely hand over sessions between 5G and 4G (and by extension 5G NSA, which essentially is 4G from a signaling point of view).

It's not a handover in the sense that you're actually creating a second connection to a different network. It's more akin to roaming than a handover.

I don't know if you're implying that the iPhone behaviour is bad but I hope not. It's obviously better.

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