That must be it!

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.

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.