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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