VoLTE has to be enabled both by the device itself (Xiaomi) and by the carrier (AT&T in your case). Even if the device supports VoLTE, if AT&T doesn't recognize or acknowledge your device, you will be stuck on 3G for calls. However, if you are getting 4G/5G data but not getting VoLTE calls, it is definitely a carrier whitelist.
You can check this by going to Settings -> SIM card -> Status -> IMS registration. If VoLTE is enabled on the carrier, it will say "registered".
My mobile carrier was kind enough to enable the IMS registration if I ask for it, but I know it's a case-by-case scenario. Try calling support and asking a manager to enable your IMS registration. I did these steps for all of the phones in my family (Xiaomi Mi 9T, Xiaomi Poco F3, OnePlus Ace 3)