You understood. You are not using VoLTE.
Historical and compatibility, I guess. That’s the way it used to work before LTE (4G) even existed (i.e. voice calls were made using 2G/3G) and when LTE was introduced, it was introduced only for data. At the time there would have been a mass of handsets that were not capable of LTE at all (so there was always going to be a period of overlap - and that period is coming towards its end).
A voice call doesn’t need much bandwidth. You don’t need the bandwidth of LTE for a voice call. Only a data call can fully utilise the bandwidth of LTE.
It is possible also that the lower bandwidth of 2G/3G is more robust in the face of unintentional interference and/or a weak signal, so maybe you are more likely to be able to make a call with 2G/3G than with 4G in marginal areas - which perhaps would have figured in the thinking of risk-averse legislators, particularly in my country where there are a lot of marginal areas. LOL.