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.
Due to marketing b*******, most things labeled as 3G are actually 2G.So networks don't support both two and three g. Then things that are actually 3G like hspda and L.T.E are marketed as four g , so it's just very confusing between all of it. They want to shut down the 2G.Networks that are edge and gsm.And they want to shut down the one g network that's cdma. Depending on your service that could be marketed as 123 or even four g.
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.