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Why most countries are struggling to shut down 2G.
(restofworld.org)
This is a most excellent place for technology news and articles.
We switched off 3G this year in the UK and my brothers phone stopped being able to make calls. He was using a 6 year old high-end Android phone, but it was from just before the cutoff where you could turn on VoLTE (calls over 4G).
Thankfully, I had a spare phone from the next year after that to hand him, and that one could work with some hidden menu (the type you type into your dialer) hacking.
~~2G is also gone.~~ Edit: it's not gone just yet. Not sure why the phone didn't try to fall back to 2G.
https://www.ofcom.org.uk/phones-and-broadband/coverage-and-speeds/3g-switch-off/
The old phone was a couple years into 4G existing but before we started to send voice over it.
I assume it just wasn't in the OS-level code. It only went up to Android 11. We could have tried LineageOS but that would have required a bunch of work including wiping the phone.
Either way, we checked and the option just wasn't there.
Were they / are they on the 3 network?
If so, that's why because 3 has never had a 2G network at all ;)