Most of the USA is VoLTE-only for a ~year already.
For other countries see https://en.wikipedia.org/wiki/3G#Phase-out
We found out the hard way when they terminated our line after having a sim card in a non volte phone for about a week, and didn't get the text alert until AFTER the line was disconnected.
The last nail in the coffin for my Motorola Razr V3