By default, the OnePlus smartphone uses the 2G and 3G network to make calls.
If we enable VoLTE (Voice over LTE), calls will be made over the 4G network when available, which provides better voice quality and faster call setup.

Note: Using VoLTE calls also reduces battery consumption.

Android 10 | OxygenOS 10.0
VoLTE calls -  Open the  Settings
VoLTE calls -  Tap on  Wi-Fi & network
VoLTE calls -  Tap on  SIM & network
VoLTE calls -  Choose a  SIM card
VoLTE calls -  Enable or disable  VoLTE
  1. Open the Settings
  2. Tap on Wi-Fi & network
  3. Tap on SIM & network
  4. Choose a SIM card
  5. Enable or disable VoLTE

OnePlus Instructions