VoLTE

2G, 3G, 4G, LTE, VoLTE, 5G all are Carrier Specific features. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you.

See the Pic below and explanation thereof

If your carrier supports VoLTE

On your iPhone, go to Settings > Cellular > Cellular Data Options and tap Enable LTE or Settings > Mobile Data and tap Enable LTE. If your carrier supports Voice over LTE (VoLTE), you'll see these options:

• Off: Turns off LTE
• Voice & Data: Allows voice calls and cellular-data use over LTE 
• Data Only: Allows cellular-data use, but not voice calls over LTE

VoLTE is on by default now. You can’t turn it off because the carriers require it for service.

That’s correct.