T-Mobile and AT&T will require VoLTE for calls after shutting down 3G
"While T-Mobile, on the other hand, doesn’t use a whitelist for VoLTE compatibility, many unlocked devices just straight up don’t support it regardless. The recently announced ASUS ROG Phone 3
does not currently support VoLTE on the network, for example. It’s not that these phones can’t support VoLTE on T-Mobile, it’s just that the device maker hasn’t done the work to outright support it. Modders on our ROG Phone II forums
have figured out the steps needed to enable VoLTE on T-Mobile, proving that a little more work at the factory could enable support.
Theoretically, as long as your Android device has a working IMS (IP Multimedia Subsystem) stack, it should support VoLTE on T-Mobile (but not AT&T because of its whitelist system.) Typical Android devices with Qualcomm modems ship with a privileged application that bridges the radio interface layer (RIL) and the IMS, which developers take bundle in their custom ROMs to enable VoLTE support
.... If the stock ROM doesn’t support it, then it’s unlikely you’ll be able to use the device on AT&T or T-Mobile once they shut down their 3G networks."