I imagine that most, if not all, modern modems (hardware, plus the software that runs that hardware) have the capability, so I would think it only requires the OEM to do the required testing and certification with the networks.
I don’t think the original intent of this thread had to do with VoLTE or non-VoLTE issues specifically. But because of the 3G shutdown, in some countries the networks have started to engage in whitelisting of devices, based on their ability to use VoLTE. [Edit: Also, a couple of countries may still be using CDMA technology, which might be a problem, at least for phones directly sold by /e/. @Manoj may know the answer to that.]
It might have been better to title the thread, for instance, as:
List of National Networks That /e/ Devices are Being Used On (Include Device Model)
[Edit 2: Another thing to consider is that models of the same device for different regions of the world may have different firmware, which is certified for VoLTE “here,” but not “there.” I experienced this problem with my phone and ended up having to flash US firmware to get VoLTE operational. This was before I installed /e/OS, however. I think /e/ replaces all that with its own firmware.
I’m currently using an AT&T-based MVNO, and my device is not certified for AT&T, so I don’t see “Enable Voice Over LTE” in my settings. When I insert a T-mobile SIM, the VoLTE setting appears, but I haven’t tested the connection, because that SIM is expired. I plan to attempt a switch in the near future, though.]
[EDIT 3, Oct 16, 2021: I have now switched to Ting, which uses T-mobile USA’s network. My device is successfully using VoLTE. (Ting can also run on Verizon’s network, but higher up the thread, people say they can’t use Verizon with /e/.)]