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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 11 | ColorOS 11.0
Step 1: Tap on  Settings
Step 2: Tap on  SIM card & mobile data
Step 3: Tap on  Calling
Step 4: Choose  SIM1/2  or  Always Ask
  1. Tap on Settings
  2. Tap on SIM card & mobile data
  3. Tap on Calling
  4. Choose SIM1/2 or Always Ask
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Oppo Instructions