When using the dual SIM feature on the OnePlus smartphone, we can also assign a SIM card to the stored contacts, which is automatically dialed on each outgoing call without the device asking which number to use.

Note: For contacts that do not have assigned a SIM card, we are asked before the call and we can also set the default SIM for outgoing calls to the phone number directly.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Contacts app
Step 2:  Choose a  contact
Step 3:  Open the  menu
Step 4:  Tap on  Set calling SIM
Step 5:  Choose a  SIM card
Step 6:  Confirm with  OK
  1. Open the Contacts app
  2. Choose a contact
  3. Open the menu
  4. Tap on Set calling SIM
  5. Choose a SIM card
  6. Confirm with OK

OnePlus Instructions