If two SIM cards are inserted in the Oppo Smartphone, the default SIM card can be determined for calls, and the defined SIM card is used for outgoing calls.

Android 10 (ColorOS 7.0)
Step 1:  Open the  Settings
Step 2:  Tap on  SIM card & mobile data
Step 3:  Tap on  Calling
Step 4:  Choose between  SIM1,   SIM2  and  Always Ask
  1. Open the Settings
  2. Tap on SIM card & mobile data
  3. Tap on Calling
  4. Choose between SIM1, SIM2 and Always Ask

Oppo Instructions
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