OPPO A57s
Android 12.0
1 Find "Number"
Press the phone icon.
Press the menu icon.
Press Settings.
Press Voicemail.
Press the required SIM.
Press Advanced Settings.
Press Number.
2 Save voicemail number
Key in 707 and press the confirm icon.
707
3 Return to the home screen
Press the Home key to return to the home screen.