“2G, 3G, 4G, LTE, VoLTE, and 5G all are Carrier Specific features. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you.”
I have called the Network Provider already as above and they do support the VoLTE.
”However iPhone model specific feature is listed here --> Identify your iPhone model - Apple Support . and tap on "See the Technical Specs for Model XXXXXX" under each model and Scroll down to "Cellular and Wireless" section.”
I have not done this one yet, but I believe Apple need to explain whether both Sims is support with VoLTE.