To install Ubuntu Touch (UT) on your Xiaomi Note 9 Pro (Global version) and ensure it works perfectly without bugs for calls, SMS, and 4G, you must use the firmware 12.0.2.0.QJZMIXM.

Download the firmware from this link: https://xmfirmwareupdater.com/miui/joyeuse/stable/V12.0.2.0.QJZMIXM/
Flash the .zip firmware using ADB sideload with a compatible recovery like TWRP:
bash
Copier
Modifier
adb sideload firmware_name.zip
It is not necessary to boot Android after flashing.
Wait for about 5 minutes after flashing to ensure proper installation.