Call barring makes it possible to block calls when roaming is activated, for example.
In the same way, you can either block all incoming or outgoing calls on your Xiaomi smartphone. Call barring can be found in the phone settings, which has the following options:

Note: If dual SIM is used, we select the relevant SIM card first.

If a password is required, please request it from your network provider.

Android 10 (MIUI 12.0)
Step 1:  Open the  Phone app
Step 2:  Open the  Settings
Step 3:  Tap on  Advanced settings
Step 4:  Tap on  Call barring
Step 5:  Choose a  SIM card   (Dual SIM)
Step 6:  Enable or disable  Call barring
Step 7:  Enter the  call barring password  and tap  OK
  1. Open the Phone app
  2. Open the Settings
  3. Tap on Advanced settings
  4. Tap on Call barring
  5. Choose a SIM card (Dual SIM)
  6. Enable or disable Call barring
  7. Enter the call barring password and tap OK

Xiaomi Instructions