OPPO A78 5G
It's not possible to make voice calls
Solution 6 of 11: The selected network mode isn't available
There may be different network modes available depending on where you are. The network mode influences the data speeds your phone can achieve.
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 Preferred network type.
Press the required network mode.
Press the Home key to return to the home screen.