We can easily make calls via the Xiaomi smartphone's clearly laid out phone app.
After entering the phone number, the virtual call button can already be tapped. If we use dual SIM on the device, the call button is split to select the SIM card from which we want to dial.

Android 10 (MIUI 12.0)
Step 1:  Open the  Phone app
Step 2:  Tap on  Contacts  or enter a  phone number
Step 3:  Tap on the  dial button  of the respective SIM card 
  1. Open the Phone app
  2. Tap on Contacts or enter a phone number
  3. Tap on the dial button of the respective SIM card 

Xiaomi Instructions