Call forwarding on the Xiaomi smartphone is set by default according to the network settings. Usually, calls are forwarded to the mailbox if "busy", "unanswered" or "unreachable".
Call forwarding can either be enabled or disabled for each case, or to any phone number.

Android 10 (MIUI 12.0)
Step 1:  Open the  Phone app
Step 2:  Open the  Settings
Step 3:  Tap on  Call forwarding settings
Step 4:  Choose a  SIM card   (Dual SIM)
Step 5:  Choose  Always forward ,  When busy ,  When unanswered  or  When unreachable
Step 6:  Enable or disable  Call forwarding
  1. Open the Phone app
  2. Open the Settings
  3. Tap on Call forwarding settings
  4. Choose a SIM card (Dual SIM)
  5. Choose Always forward, When busy, When unanswered or When unreachable
  6. Enable or disable Call forwarding

Xiaomi Instructions