OPPO A78 5G
It's not possible to make voice calls
Solution 4 of 11: The selected network is out of range
You can set your phone to select a network automatically or you can select a network manually. If you select a network manually, your phone will lose network connection when the selected network is out of reach.
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Mobile network.
Press the name of the SIM.
Press Operator.
Press the indicator next to "Auto-select".
Press Off.
Press Off to turn off the function and wait while your phone searches for networks.
Press the required network.
Press the indicator next to "Auto-select" to turn on the function.
Press the Home key to return to the home screen.