@Ida_ Thank you for pointing me to the thread regarding VoLTE success stories. Unfortunately, my carrier is AT&T, which apparently is stricter in what it allows than some other carriers. For example, the only U.S. success I see in that thread is using a Xiaomi phone, which is not in AT&T's compatible phones list. I appreciate your further search suggestion too. Maybe that will lead me to an appreciable solution in the future, though for now I assume the Ubuntu Touch "Supported Devices" list includes all that are really prepared up to the stable standard I need.