OPPO A16s
Selecting ring tone
You can select the ring tone you want to hear when you get a call.
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Sound & vibration.
Press Ringtone.
Press the required SIM.
Press Select from files.
Press the required ring tones to hear them.
Once you've found a ring tone you like, press the Return key.
Press the Home key to return to the home screen.