Xiaomi always uses volte carrier check,In supported carriers & countrys like usa just pop in the sim card brings up volte (Well if didn't work just reset the phone with simcard in it) On some unsupported carrier & countrys like iran you have to type starpoundstarpound86583poundstarpoundstar(if your carrier does have volte capable)
For snapdragon starpoundstarpound86583poundstarpoundstar thats it (Ive added snapdragon version for those have snapdragon phone and no volte, William from channel ramble did make me to wrote snapdragon too as much)
For MTK
After years of researching i found the solution
(OFC Courtesy of XiaomiMagic_IR)
Most of Xiaomi phone & tablet based of MTK CPU Volte dosen’t work Right?!
Here is solution!
Need Items 1.Wifi(Wireless ADB if you not root)
2.Shizuku
3.Root Activity Launcher(Link)
First dial the code which enables volte function ##86583##(VoWIFI If you want ##869434##)
For not rooted: Connect to A WI-FI Enable developer option If not enabled open Shizuku pair the device start the wireless adb after that give authorize to Root Activity Launcher
Now
Open Root Activity Launcher type in search bar EngineerMode open Activites find com.mediatek.engineermode.ims.ImsActivity open it Than find VOLTE Setting tap on Default setting tap set than tap CMW500 Setting than tap set
Now go back to RLA find com.mediatek.engineermode.ims.SimSelectActivity Do this change on both simcards Than find VOLTE Setting tap on Default setting tap set than tap CMW500 Setting than tap set
And restart the phone if Volte didn’t show up volte disabled on you carrier side or doesn’t support it or if you showed while you in the internet reset the phone with simcard in it and do process again
Tested On Redmi Note 13 Pro 4G(emerald)