VoLTE, by default is not enabled on most Xiaomi phones, Poco F2 Pro included.

If you want to enable VoLTE, check your
• Settings
• SIM Cards & mobile networks
• Your Telco (eg. Digi)

If VoLTE is not stated in the page, your VoLTE is not enabled.

To enable VoLTE, go to your dialler & type
*#*#86583#*#*

You will get a message that "VoLTE carrier check was disabled"

Now go back to "Settings/SIM card.../Your Telco" & you will see the VoLTE toggle button appearing: button slided to the right means VoLTE is now activated, and a small notification will now appear on the top left notification bar.

NB: please note not all Telco has activated VoLTE services

so far, Digi, UMo & Unifi 4G are the ones that is confirmed you get VoLTE.

Maxis is rolling out in batches and very, very slowly.

Celcom... not very sure though.

so, as per earlier replies, it depends on your telco.