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By default, calls are made over the 2G or 3G network, depending on reception. After enabling VoLTE (Voice over LTE) on the Oppo smartphone, calls are made over the 4G data network, which improves voice quality, speeds up call setup and reduces battery consumption

Android 11 | ColorOS 11.0
Step 1: Tap on  Settings
Step 2: Tap on  SIM card & mobile data
Step 3: Choose a  SIM card   (Dual SIM)
Step 4: Enable or disable  VoLTE Calls
  1. Tap on Settings
  2. Tap on SIM card & mobile data
  3. Choose a SIM card (Dual SIM)
  4. Enable or disable VoLTE Calls
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Oppo Instructions