If a phone call is in progress and another call is received by the Xiaomi smartphone, a notification can be displayed. This is done both visually and acoustically.
In the phone settings, Call waiting can be switched on/off.

Note: When using Dual SIM, the call waiting function can be (de-)activated for each SIM card individually.

Android 10 (MIUI 12.0)
Step 1:  Open the  Phone app
Step 2:  Open the  Settings
Step 3:  Tap on  Call waiting
Step 4:  Choose a  SIM card   (Dual SIM)
Step 5:  Enable or disable  Call waiting
  1. Open the Phone app
  2. Open the Settings
  3. Tap on Call waiting
  4. Choose a SIM card (Dual SIM)
  5. Enable or disable Call waiting

Xiaomi Instructions