Yes, it is possible to make phone calls using LTE network only. This technology is called VoLTE.
It indeed uses data transfer under the hood but the data is being routed differently than "normal" data transfer and therefore works without a need to have cellular data connection enabled. And this is the technology which will be somehow "required" in T-Mobile US and it is being integrated in many LTE networks all around the world.
An equivalent for 5G is called VoNR (Voice on New Radio) or Vo5G.

It is called HD Voice. And as far as I know it can be used even on old 2G network but US operators lauched it alongside with LTE. Perhaps so that the user can "hear the LTE improvement".


P.S. There is also LTE Broadcast (also called eMBMS, for 4G) and 5G Broadcast (also called FeMBMS) for transmitting (emitting, multicasting) data (f.e. TV, radio) using 4G or 5G with possibility to receive data without any additional charge and even requiring the SIM card. But, unfortunately, it is not popular - there were just few public tests of these technologies.