What are the factors affecting the ability to enable/disable VoLTE?

In response to this [zero-day vulnerabilities for certain Exynos chipsets](https://9to5google.com/2023/03/16/google-exynos-modem-vulnerabilities/) , I tried to turn off VoLTE on Samsung Galaxy S9 running Android 10 (build number QP1A.190711.020.G960USQU9FVB2) but couldn't find the setting under Connections -> Mobile Networks. This phone runs on the USA AT&T network. (Thanks for the tip that G960U doesn't use Exynos; even in G960F which does use it, the model is not affected.) But I did find the setting to turn it off in another Samsung Galaxy phone, an A20 running Android 11 that runs on the BoostMobile network (which I believe uses the AT&T network as an MVNO). Again, although this phone [does use Exynos 7884](https://www.gsmarena.com/samsung_galaxy_a20-9640.php) it may not be vulnerable, going by the [list of chipsets provided by Samsung](https://semiconductor.samsung.com/support/quality-support/product-security-updates/) . **My question**: What are the factors affecting the ability to enable/disable VoLTE? Is it based on the OS version, the chipset, the provider, or the location (dynamically, based on the presence of LTE there)? It seems that [for some MVNOs it cannot be turned off](https://www.androidcentral.com/volte) ? Another way to pose the question is in terms of technology. In areas where 4G is the only network (since [both AT&T and Verizon would have turned off their 2G and 3G networks by now](https://www.fcc.gov/consumers/guides/plan-ahead-phase-out-3g-cellular-networks-and-service)) , **do ALL voice calls over 4G network are necessarily voice over IP**? If not, does turning off VoLTE means simply that you cannot use data while making calls?