That's a good option for regions where GSM/2G or 3G still operate concurrently with 4G/5G. I use that solution with my Pro1 in Germany.
However, @Name_not_avail is located in the US. They recently shut down all pre-4G networks. 4G simply does not implement traditional voice calls outside of VoLTE. Hence, with the modem no longer able to fall back to one of the older, non-IP-based protocols (2G/3G) upon initiating or receiving a call, VoLTE is now mandatory in the US.